Firefly hit the nail on the head for me, too; there are many things not to do with pain that I will endure for his pleasure.

I enjoy pain (of course, I have my limits and preferences). I will endure pain that I don't enjoy for his pleasure - it is a way of giving myself to him, and despite not enjoying the pain, I enjoy the act of giving very much. However, if it is a pain I don't enjoy then I certainly can't take as much of it.