Beta-endorphin. A form of chemical produced naturally by the body that seems to be involved in regulating the perception of pain. The name is derived from 'endo-' meaning internal, and the name of the drug morphine: endorphins are chemically similar to opiates, and one of the factors in opiate addiction is that the body stops producing them in response to the presence of the drug. The detached euphoria sought after and enjoyed by some SM bottoms as a result of heavy pain games may in part derive from the release of endorphins.
My introduction to endorphins were with my first tattoo. Why did I want more the minute it was over, after counting the seconds while it was happening? Then testing my limits with longer tattoos sessions. I never imagined how sex and control could enhance those feelings even more. The addiction to those feelings are real. Wanting to push yourself more and giving up control to your master... is life changing.