Bathroom Conversation

There are certain classes and groups of people that absolutely fascinate me.  One such group consists of those who find the time and opportunity to write upon the bathroom stall walls.  I try not to analyze this behavior to closely but just how does this scenario play out?  They're sitting there on the can, feeling bored, luckily they happen to have a black sharpie in their pocket, and then suddenly they get the impulse to draw up some gang sign or wiener joke?  Or is the whole thing premeditated, and its just a artistic hobby for some- going from one  public bathroom to another treating the stall walls as their personal canvas?