Why do I hate sex, and how can I change it?

Nicole
I am 17 and have been with my boyfriend over two years. When we first met, he being older than me I just went along when our physical relationship developed quite quickly. I can't say I didnt like it, but it was always him initiating. 
We had some problems later on and the physical intimacy stopped all together. I have always believed in abstinance anyway and I really really really REALLY don't want to get pregnant so I used this as an out and we just did not have sex. 
Now, problems resolved for a long time, we are emotionally closer and happier than ever. He's my best friend and I love him very much, but he has recently been asking to start being physical again and I can't.
I don't know why, but I don't desire sex. I have enjoyed it in the past, but that's if I can get to that point. Most of the time simply being touched or kissing for too long just makes me feel... Uncomfortable I guess. I've never really desired sex, but this is worse. I can't be physical without needing to pull away.
I don't like this. I mentallly want to have sex with him and be that close again, but I cannot physically bring myself to it. What can I di about this???