I'm not a huge animal rights style person, I'm doing it for different reasons, including:
1) Meat weighs me down too much. It sits in my stomach/intestines too long and makes me tired. My body must be working harder to process it - if I eat a burger for lunch, I'm sleepy and dead forever. Veggies perk me up.
2) Being a veggie forces me to think more about what I eat. It eliminates so much fast food from your diet that you end up eating better. When I've been injured, I don't gain so much weight since I went veggie.
3) Eating lower on the foodchain is better for the earth. Can't stress it enough. This will become a huge issue as the population continues to grow.
4) I'm terrified of Mad Cow Disease and a few other nasty's spread by eating meat. Read "mad cowboy" or "the jungle" or "beyond beef" and you'll see what I mean.
5) Many, many, many other reasons......