Perhaps it's less to do with the injection, and more to do with the loss of control to someone you trust. By default, you are less likely to trust a nurse with using a needle on you, than you are a needle fetishist that you know; despite them both having training. I'm wondering if you have counterphilia, which means you are sexually attracted to fearful situations, which, in turn, implies masochism. Specifically, the love of injections or needles puncturing your body is called trypanophilia.