I'm a vegetarian (vegan on and off) because I'm an animal lover. I try to go vegan sometimes but I have difficulty sticking with it. My motivation for going vegan is because of the animal rights issues at hand as well as the hormones and antibiotics being used on dairy cows, plus there's the connection to veal...
I try, but I love cheese and ice cream, so I give in sometimes.
My best friend had cancer, and
had to go vegan for a month before a procedure. Her doctor gave her "The China Study" and she passed it along to me. Fascinating stuff!
Another contributing factor was mercury poisoning. I never had it, but I read an article years ago about it contributing to problems with autism. I don't know if this is true or not, and I don't even have kids, but the idea that what I eat could damage my future children scared me. Again, I have no idea if there's a correlation or not, it just stuck in my head.