Money is power in human society, and just like other animals we seek power over others - to be the "alpha".

At least, that's my theory, simply put.

Edit: To add to that, we are of course human, so there's people who choose to ignore that alpha drive and instead be giving and kind. I'm definitely not saying we're all like that (I'm sure as heck not), but I am saying I think it's one of those "by nature" things that some choose to do.