It is, of course, the brain. When you think about it all the practical stuff involving body fluids is only fun because the brain recognises it as fun. To survive as a species we need to reproduce, which is why our brains are hard wired to enjoy sex, and to want to have sex.
If you think back to when we lived in the jungle, running away from a tiger or running after something edible was critical for survival. Keeping going even when we were terminally tired out and in pain even more so. The way I see it the brain learned to shunt the pain sideways into pleasure when something had to be done. That's why athletes get an endorphin high after doing something amazing.
That's why BDSM is pleasure, in the right context all sorts of pain can be shunted into getting a real high. But the context depends on there being a caring relationship behind it.
