i wanna break up with my boyfriend

he's so nice and supportive and caring, but I just don't feel any attraction to him anymore. I don't know why though. he's even been going to the gym more often so he looks so much better than when we started dating last year. I just feel annoyed at him anytime he tries to talk to me or do anything nice for me. I feel like I'm just being a bitch now and I don't know what to do.