Rough Sex: Why Is Pain Pleasurable?

I notice something about myself when I got my last tattoo, the pain I felt when getting it was so intense, that at first it just made me wet, but when the area was numb to the pain all I could feel was the vibration which in turn made me cum. Don't laugh, because I know you probably are, but seriously I had this sense of euphoric pleasure. I didn't cum so hard that I arched my back like I would during sex, but I came hard enough where the tattoo artist had to stop and wait for my body to stop moving so he wouldn't mess up. It has however come up more and more in the bedroom department. Maybe its my desire for the man i am with to have more aggressive behavior, but the spanking, the pulling of my hair, makes the sex all that much better. The adrenaline rush from the pain makes it hotter and thus much more enjoyable. So where does this desire originate? any suggestions? Not that I am complaining, just a little curious.