Why can't i feel anything?

Ok how can i get more in the mood when my boyfriend and I have oral? Just liek when we do it, i'm totally never in the mood for some reason, and i have never had an s and he knows this and it makes him sad soo we dont do it that much anymore, i want to do it, but it's like when he does it, it's like i'm num, i don't feel anything. Whats wrong with me? I've had with a chick and it was better then this lol. Is there anything i can do? To have me reach that ?