Question about sex......?

My boyfriend doesn't seem to be as interested in me sexually as he was in the beginning of the relationship. Everything's going great, he loves me more than ever and I can tell, there's no doubting that. However, we started dating two years ago and in the beginning everything was all attraction and arousal, and now he's just not that into it anymore. We have sex and it's great except for the fact that I feel like he just does it because he knows how much I want to. I'm an attractive girl and I know it, so whats up?