Coworker slapped my butt?

ok, before anybody screams sexual harassment, I like the guy and we play around all the time, so I'm cool with it.

i was slightly bent over reading the paper, and he came up and smacked my butt with his briefcase. So, in a guys mind, what makes them do this/ I'm just curious, he's done it before but its been a couple years. Did he do it because he just saw the opportunity that I was sort of bent over, or what? thanks!
Updates
+1 y
Also, how can I flirt with him and tell him I'm OK with it and enjoyed it?
Coworker slapped my butt?
Post Opinion