Numbers next to names

Why is it so compelling to maximize a number next to your name?

I first noticed this phenomenon on a casual games site I used to work on - each player had a rating for each game and people were fiercely attached to that number. You see it on WoW with the fixation on level. In GW, the equivalent is probably the rank & fame system.

I'd be the first to admit that I suffer from this as much as anyone. This comes up because some of the trolls on /. gave me positive moderation - on what was, in retrospect, a well-timed karma whore quote post - and it gave me a happy warm glow. Why do I care?

I'm really not sure. Evolutionary psychologists in the audience, you're up.