@RachelW I often wonder the same things. But here is my observences from playing with my partners.
As a sensual sadist I do not enjoy inflicting pain, I call it inflicting pain in a very pleasurable way. What my partners have explained to me about the desire to recieve pain to pleasurable or erotic areas is that they have predisposed them selves to not seeing pain as a result. But to get there they have to feel particular connection to the person administering such attention.
Pain and pleasure are not mutually exclusive. I have found them to connected. The things we do with each other given the normal wiring is not sexy or pleasurable. In a binary state it’s just pain. Much like how sex isn’t entirly pleasurable without forplay. We will accept much more when we have been stimulated to readjust our mental and emotional senses so that we can recive pain as a sensation. (I stole that line from a mentee of mine)
It then becomes an entire body experiences and their synapsis are firing on all cylinders so that by the time we get to a senstive area such as the clitoris the acceptance and desire has already been set.
Why do my partners enjoy it, they tell me it’s because they feel safe, protected and a full power and neregy exchange has been established. The nned then isn’t about thje pain it’s the sensation of actualizing pleasure.
That’s what I’ve heard. And I’m sticking to this story and that’s either going to get me spanked or hear rubs. 
