Pleasure and Pain.

Mad Science: Neuroscience Explains Why You Get Pleasure From Hurting Yourself:

"It turns out there is a neurological explanation for why people scratch and cut themselves, and spank each other for pleasure. Inflicting small amounts of physical pain, whether from scratching your skin vigorously or doing something more extreme, deactivates the parts of your brain associated with unpleasant or painful emotions. Though scientists have long speculated that there was some kind of neurological payoff from self-inflicted pain, a study published yesterday demonstrated precisely why your brain gets a reward when you hurt your body.

...It's also possible that the pain of scratching, or more intense pain from cutting, suppresses painful memories too."