Many people claim to experience an endorphin rush after undergoing modifications.
The human body releases endorphins. They are commonly associated with intake of tryptophan (commonly found in chocolate). Endorphins are the body's "happy" hormones, however, they are not responsible for the effect. The supposed "endorphin rush" is more likely to be a burst of adrenaline (epinephrine), with the side-effect of numbing pain. This is a stress response, generally evoked in response to pain or fear. It is a defense mechanism.