Well I've never ever liked red meat. As a kid, I remember being told by my mum that I couldn't leave the table until I'd eaten more of my dinner...she'd turn her back and start washing up and then I'd start stuffing sausages or whatever it was in my pocket, hide it under other foods or feed it to the dog. Just anything besides eating it!
So really that left me with chicken and fish. But last year I started looking at the chicken on my plate and found myself repulsed by it. Just thinking that I was chewing on something that was once a walking, breathing, living thing made me feel sick. I began to feel it was wrong after all killing another human is wrong...you're taikng a life that's not yours to take and I couldn't see how killing a chicken was different.
So yeah, that's what started it but over time other reasons such as environmental concerns, animal welfare, health have started to play a part.