Personally, I do it cause I can and I like it.
Person 1- Look, my new hamster.
Sadist- Mate, that's not a hamster, it's a piece of flufffy poo.
I can't believe you wasted good money on that piece of crap.
Person 1- You're such a sadistic prick.
Person 1- Aah man, My arm seriously hurts.
Person 1- Yeah, I just smashed it off of the door frame.
Sadist- So does it hurt if I do this? *hits his arm*
Person 1- Ow, yes it does.
Sadist- Yeah I thought it would.
Person 1- You sadistic bastard.
Masochist: HURT ME!!!