look again On Spanking

Humans engage in diverse activities that create pleasure and fun from "stress". Horror films, rollercoasters, pranks, rough sports, caving, bungee jumping and so on.

Some research links their pleasure (rollercoasters) to the cortisol release. But their stress is controlled stress--a tease since while one of the brain is put in fear or distress, another knows it is safe. The experience is positive--excitement. The same applies to S&M.