Why do you go to a cash machine?

A weird thing happened last night - I was at the end of a queue for the cash machine and when it came to my turn I found that the guy who had used it before me had left a big wadge of cash in the machine.  Of course, I called the guy back and pointed out his mistake.

There's a problem with that last statement - the "of course" bit.  There was no-one behind me.  The guy was about to walk off and cross the road.  I will admit that there was a brief fast forward video in my head of me grabbing that cash, jumping in my nearby car and speeding off.  This was accompanied by a couple of parallel thoughts.  How much is there?  What kind of bass could I buy with that?

Thoughtcrime?

I'm glad I did the right thing, but don't get me started on the reasoning behind it because that's whole new box for Pandora to open (did I do it because it's the right thing to do, or did I do it out of fear of the consequences of not doing it?)

What the hell is this?  Psychology?  Sociology?  Criminology?  One thing's for sure - I never thought going to the cash machine could be so thought provoking :)