For me, it's a moral thing. I've thought about it quite a lot, and I decided that I don't think eating meat or using animal byproducts in general is immoral. Certainly the industries that produce them are terrible, but at the basic level, I think it's completely justifiable to end another animal's life to lengthen your own. That said, I definitely think it's a better choice to rely as little on those kinds of things as possible. I decided this about four years ago after talking to a friend really got me thinking, and I decided that because I was in a position where I could go vegetarian (I don't live in a household that only eats meat, I don't have allergies that would make it very difficult to function on a vegetarian diet, etc.) I should go vegetarian. It was the right choice for me. It wasn't an incredibly difficult thing in my situation, so I felt like I should make that change.
My living situation changed very recently, and (long story short) I'm now preparing my own food where in the past I was generally not doing so. I decided that since I was finding my own food anyway, this was the perfect time to choose what's in it and take the step to go vegan. I don't know if I'll stick with it (and it makes it incredibly difficult to eat out), but for now it's a good choice for me. I'm considering going back to vegetarian, but only cooking vegan food (vegan food at home, vegetarian food at restaurants) or something like that. There were also some health motivations, but I don't know that much about health, and that part was more along the lines of, "Hm, maybe this will be good for me." The main thing for me is morality.
What about for you? What were your motivations?