I became a vegetarian in early 1997.
It's not for health reasons. Though like most vegetarians I've found that I eat much healthier food than before. Though ironically I find that my health hasn't been as good - I think some people are particularly suited to vegetarian diets, and I am not.
So around 2002 I started to eat fish. I also eat cage-free veggie fed eggs, but I strictly avoid milk for a large number of reasons. Technically this makes me 'ovo-pescetarian'
It's not for religious reasons, I'm not exactly religious.
It's because I love animals, and decided it wasn't particularly nice to eat them. I think it's our responsibility to eat as low on the food chain as we can.
"I have no doubt that it is part of the destiny of the human race, in its gradual improvement, to leave off eating animals, as surely as the savage tribes have left off eating each other when they came in contact with each other."
- Henry David Thoreau
"We need another and a wiser and perhaps a more mystical concept of animals ... We patronize them for their incompleteness, for their tragic fate of having taken form so far below ourselves. And therein we err and err greatly. For the animal shall not be measured by man. In a world older and more complete than ours, they move finished and complete, gifted with extensions of the senses we have lost or never attained, living by voices we shall never hear. They are not brethren; they are not underlings; they are other nations, caught with ourselves in the net of life and time, fellow prisoners of the splendour and travail of the earth."
- Henry Beston 1888-1968