This is where most cutters do it. Cutting is about the pain. The pain helps them with stress and the adrenaline boost is kind of addicting. I don't do it intentionally, but when I bust a knuckle while working, the rest of the job seems to go more smoothly. Pain is a strange motivator like that.

There are better methods to cope, and usually cutters do it because of stupid first world style problems. That is, they aren't busting their knuckles doing anything useful so they kind of crave some real pain to drown out the petty bullshit that fills their lives.