Why won't I let my boyfriend touch me?

Every time he tries to touch me (ually) I automatically push him away. I have no problem touching and doing stuff to him, but for some reason I won't let him do anything to me. We haven't have yet, but we've done other things that involve me doing the work. He's tried many times to do things to me but I just can't. Is there some sort of subconscious reason why I do this? It's so frustrating because I don't want him I'm purposely pushing him away every time. What can I do to stop pushing him away? I'm 18 and he's 19.