(Most info here is from T. Campbell, P. Singer, or M. Pollan's works iirc; if you want to know specifically where something comes from I can dig up the references, but that's a lot of work and this is just what I've synthesized.)

While preparing for the GRE, in one of my characteristic attempts to avoid Actual Work, I started reading up on veganism. I've been a vegetarian for about a year and a half, and pseudo-vegetarian for about six months before that, and that's been going pretty well; in Ann Arbor, lovely town that it is, vegetarianism is not especially uncommon, so it hasn't really been difficult or shocking to explain to people. However, when I started telling people I was going vegan, I was overwhelmingly met with incredulity and consternation. I have also been met with some common questions; I don't mind answering them at all, but as the school year is beginning shortly I thought I'd post a manifesto of sorts to save everyone time.

My Reasons:

1. The environment.
It should come as no surprise that the business of producing meat is not good for the environment, on several levels. The contributions of livestock production to greenhouse gases, ammonia emissions, and land degradation is hardly minimal, even in the best situations. Not only does the livestock industry consume vast quantities of water, but it is also one of the largest sources of water pollution, thanks in large part to animal waste and various chemicals involved in keeping animals alive under nightmarish conditions.

2. Animal rights.
Putting aside the question of whether it's ethically sound to kill another sentient being, the sheer amount of suffering and pain that goes into every bite of steak or sausage or drumstick is staggering. With the exception of sustainable and humanely run farms--and let me assure you that they are the exception--livestock live violent, brutish existences suffused with constant pain and crippling monotony. They are forced to eat food their bodies cannot handle--mostly corn and animal products--and to compensate are drugged up to the gills.
It never fails to amaze me that we as a society can get so outraged over something like dogfighting and continue to contribute to this much vaster and sharper cruelty without any sense of internal contradiction. Pigs, for instance, are on the whole considerably more intelligent than dogs and can certainly suffer just as much, but what we would call unconscionable animal cruelty to a dog is perpetuated against countless pigs every day.

3. Health
The tortuous conditions livestock are kept in do not produce healthy meat, and that gets passed down to the consumer. As the saying goes, GIGO--garbage in (quite literally in this case), garbage out.
Furthermore, the importance of meat itself has been vastly overblown. There is literally nothing in meat that we actually need which we cannot get elsewhere, especially in this day and age--we can go to the farmer's market or supermarket or anything in between and find a staggeringly diverse range of plant matter. We have this fantasy of meat as a vital substance, but it is not life we consume--it is death.
I won't get into the extensive health costs of meat, but there's plenty of research available for perusal; a good start is T. Campbell's 'The China Study.'

4. Economic stability
I will be the first to admit I am not an economist. Nonetheless, it is not difficult to see that meat and meat by-products in America are a drain on government funds.
The question we ought to be asking is: why is meat so cheap? By the most conservative estimations, beef costs more than 30x in water than wheat, pound for pound. Once we factor in the cost of land to grow feed, the antibiotics and other chemicals pumped into every animal, and the energy cost involved every step of the way, the costs ought to be astronomical. Most of the energy that gets put into an animal is burned off by its metabolism; we pour money into animals to maintain a stasis. By all rights, meat should cost at least $10-20 a pound.
The fact is that cheap meat gets paid for somewhere down the line. Some part of it is paid in our environment--we trade clean air, fresh water, and fertile soil for cheap meat. Some part of it is in quality--it is profitable for meat manufacturers to churn out unhealthy meat to a country that by and large cannot or will not tell the difference. But a large part of it is the billions that the American government forks over to produce meat, milk, and eggs every year. There can be no justification for such a gross misappropriation of funds. The subsidies that are paid to meat manufacturers are not for the good of the people, they are for the good of the meat industry.
Also, on the whole plant matter is still cheaper than meat, so it's good for my wallet.

Common Objections:

NB: I myself held every one of these objections at one point or another. Nothing in my culture or upbringing primed me for veganism, and I am all too aware I am not alone in this. There's a lot of misinformation floating around about animal products and veganism. I hope to address at least some of these misconceptions here to the best of my ability and understanding.

1. It's natural for humans to eat meat.
1a. Other omnivores and carnivores eat meat, so why shouldn't we?
The meat we're talking about isn't game we've hunted ourselves in a need-based system--it's a product of many, many generations of controlled breeding in controlled environments. There is absolutely nothing natural about domestication.
1b. We're adapted to eat meat--check out our canines, etc.
Yes, we are anatomically equipped to eat meat. But just because we can doesn't mean we are obliged to. We are, for instance, physically capable of murder but it does not automatically follow that we are given an ethical carte blanche to go on killing sprees. Moreover, our digestive systems and teeth are really only equipped to handle well-cooked meat. Consider our relative the chimp: although chimp teeth and jaws are much more developed than ours, they still have difficulty processing raw meat.
1c. Meat and milk are healthy and necessary for us to be healthy.
We are perfectly capable, as a species, of obtaining all the nutrients we need from plant matter--especially with the dazzling variety available today. Moreover, there is plenty in animal products that actively impedes good health. We are thousands of years away from the times when meat was simply the easiest way to get certain nutrients.

Sidebar: a variation I hear a lot is "But milk's natural, right?" Imprimis, when we say 'milk' in this culture we pretty much invariably mean 'cow milk.' Humans aren't 'meant' to drink cow milk, and never have been. Second, tolerance of milk in adulthood is relatively recent in our evolutionary history, and even so it's very culture-dependent. For instance, many Asians (like myself) have some degree of lactose intolerance and yet we're generally considered human; it's not natural for me to drink cow milk, and my body concurs.
Also, not only is cow milk not the only source of calcium available but studies show it may not be all that great a source to begin with. Got Milk? is nothing more than dairy propaganda.

2. Their purpose is to be eaten.
2a. If we don't eat them they'll overrun the planet.
This is pretty much the opposite of what would happen, but a surprisingly common assumption. We are responsible for these animals at every stage of their life, including their creation. If there is no demand, supply will drop off under the conditions of a free market.
2b. If we don't eat them they'll go extinct.
Also known as [link] This argument overlooks several key facts, most notably the distinction between domesticated animals and wild animals. As always, domestication changes everything. Also, the argument ignores the primary reason many species go extinct: humans. Nobody is saying "Oh, it's perfectly okay to destroy this habitat if you're going to drill for oil, but don't put a factory farm here," or "Absolutely I support sport hunting! Just don't eat it, all right?"

3. Eating meat makes me happy.
I hear heroin's pretty good for that, too. Anyway, something I've found--and something I think a lot of vegans will tell you--is that once you're off the stuff for a while, it starts looking and tasting rather unappetizing.
Variation: But cheese is delicious.
That's the addictive and highly concentrated casein talking. Just Say No.
I used to be crazy about cheese; it was the main reason I cited for not going vegan. Then I did some reading and was pretty much horrified at what I was putting into my body. I don't like the idea of being controlled by food, and I have a tendency to be a bit ornery when I think I'm being manipulated.

4. Plants are alive too, so what's the difference? (or, more sarcastically: What about plant rights, huh?)
Even assuming plants have the ability to suffer on anywhere near the scale that animals do, when you eat an animal you're eating all the plants it ate too--and getting a fraction of the benefit. We can't live without plants, but there is absolutely no reason not to live without animal products.

Common Questions:

1. Blah blah PETA blah?
No. Just no. PETA can go straight to hell with their misogynistic, fatphobic, racist, attentionwhoring ways.

2. How do you get enough protein?
Getting enough protein is exactly as important for omnivores as it is for vegetarians/vegans. As long as an individual consumes a variety of foods* protein shouldn't be an issue, whether their chosen foods include animal products or not. In fact, omnivores ought to worry a good deal more than they do about ingesting too much protein; on average, they get a lot more than they need and generally more than is healthy.
The only particular thing vegans ought to watch out for is B12, but that's in dark leafy greens so as long as vegans keep kale, chard, collard greens, etc. in the rotation (and/or maybe a supplement) everything should be fine. And who doesn't love kale?

3. Don't you miss ___?
I did a bit, at first, but overall it's been quite an easy transition. I haven't really gotten cravings since I started getting a balanced diet. Part of it is that I've discovered such good food since going veg; the other part is a bit more complicated. As a number of you know, I was in Roswell, NM, for six weeks this summer. I managed to stay vegetarian, but almost every food item there was loaded down with overprocessed ingredients; I pretty much existed on yoghurt, cereal, and cheese. Heavy on the dairy and grease, low on everything else. I did find some soy milk at the local Target; I don't know what I would have done without it. What I missed was food. Now I have food, real food, and I don't feel any pressing need to give that up.
Ultimately, after looking at the available evidence, I made a choice not to be the kind of person who could stand to eat meat products. I'd like to say it was a more purely altruistic motive, but what really stops me from reaching for the steak knife is equal parts care for the aforementioned reasons and a refusal to let myself be someone who can knowingly contribute to the destruction of my health, of my fellow creatures, and of my planet.

4. Is this going to affect me in any way?
No. I'm not going to force-feed anyone sautéed eggplant and sweet potato fries, nor am I going to insist on vegan restaurants etc. A number of my favourite restaurants in A2 happen to be vegetarian/veg-friendly, but I've liked them since before I even went pescatarian so I'd like to think I'd suggest them anyway. If I cook/bake something for you, it'll be vegan, but you're not obligated to eat it or like it just because I do. So, y'know. I'm not going to throw paint on you or anything. You can stop looking so worried.

If any of this makes sense, here are a couple suggestions.
1. Take a look at how much you spend on meat every week or month. Consider buying less amounts of a higher class of meat products--i.e. humanely treated organic meat products--with the same amount of money. Yeah, it seems more expensive, but when you consider the value you get back in health alone (not to mention quality of taste, toll on the soil and air and water, and animal suffering) you might consider it a bargain.
2. In the same vein, try committing to eating vegetarian one or two dinners a week. You probably do already, but personally I've found that making that conscious connection helped me see the amazing diversity of food available once I stepped out of that 'must have meat or it's not a real meal' mindset. It's not a huge commitment, but it gives back a lot. The more 'real food' you realize you're eating, the more satisfactory your eating experience will be.

Effects on My Life:

I've noticed a few things in the course of vegetarianism/veganism.
The biggest change has been in my relationship with food. I've always liked cooking, but for one reason or another I subsisted largely on packaged food during my first couple years of college. It was gross and unhealthy and I ended up abandoning the concept of a meal and simply reaching for food a few times a day. That was no way to live, and I knew it.
Furthermore, I--like so many Americans--couldn't get over the idea that food had to have meat in it in order to 'count' as a meal. Vegetables, fruits, and grains blurred together in my mind into featureless and flavourless accessories.
Vegetarianism meant I had to start thinking about food; I had to work towards a real understanding of what food means. Where it comes from. Its roles and its consequences. And the simple act of shifting my gustatory focus away from meat products unfurled a world of culinary possibilities and broadened my horizons.
So far, veganism has taken this visceral** contemplation to a dramatically deeper level. I'm reading ingredient lists and not liking what I see. I'm researching real nutrition, and I'm caring more and more who and what was involved in the production of my food. I'm getting more and more involved myself; I'm searching out more raw ingredients, trying to make more things from scratch. I'm no longer separated from food by chemicals and preservatives and fillers, and am increasingly sensitive to the nuances of flavour. With just ten to twenty minutes of effort, every meal becomes a way for me to connect with myself and the world that is constantly renewing me.
This vegan thing still sits pretty new on my life--otherwise I wouldn't be writing this--and we'll see if it lasts, but I have a good feeling about it.

*Note that overprocessed fat/corn/sugar-laden foods don't count as variety, and you shouldn't be eating them in any case.

**ahahaha, get it? ...yeah, you can ignore that. I can't help myself, it's like a disease.