Had to throw my hat in the ring on this one... I haven't read all of the postings on the subject, so I'll apologize now if re-inventing the wheel. My guess is that any activity that a person finds pleasurable involves the release of "endomorphs" which in turn produce a natural "high" -- a state of physical and mental well-being. Maybe we're talking about some kind of surrogate neurotransmitter?? (As you've probablyalready concluded, I'm not a doctor -- I don't even play one on TV ... but I'm having fun.)