A short list, by the way, of some of the reasons I'm glad I've gone vegetarian:
-drastically cut back my participation in antibiotic use on animals
-drastically cut back my demand on limited agriculture land
-save plenty of money
-beat the "choice paradox" when eating out and enjoy myself more when I'm content to order whatever is vegetarian on the menu
-appreciated vegetables infinitely more, now that they moved from the side dish to the main dish (I hazard that if a vegetarian may lack some B vitamins, your average omnivore almost certainly lacks vitamin A and many others found in vegetables)
-learned significantly more about personal nutrition
-got better at cooking, and ate in more
-clean shits, and regular
-an interesting conversation piece at communal meals, often more pleasant than you might expect
-girls like it
-lower risk of cancer, obesity, heart attack, stroke
-when you meet another vegetarian you are instantly friends
-no more putting thoughts of crying, squealing, human-like screams out of your mind at the dinner table
That's just what I can think of right now. Downsides are sometimes old people glare at you, and I suppose I may not be as flabby and muscly as I would if I still ate nasty lunchmeat, and sure, I'll even allow I may not get as much B vitamins as I ought, but I am pretty careful about such things, owing to the increased awareness of nutrition than comes from switching diets.
So there you have it. My actual reason for doing it is ecological. The share of meat Americans eat, extrapolated to everyone, would necessitate 6 or 8 earths. The dead zone in the gulf that is fucking over the fishing industry on my beloved Gulf Coast is caused by fertilizers running off Midwestern fields into the Mississippi. Much of this is for use as animal feed and ethanol fuel, both of which are inefficient and unnecessary. The animal rights angle is less important to me, but certainly I can rest easier, especially with respect to pigs, which are so similar to humans in some eerie ways.