QUOTE (Blade @ Nov 13 2009, 04:29 AM)

IIRC, it rewards curiosity (new experiences) with pleasure.
Actually, the brain works mostly this way: it rewards you with drugs when you do something it wants you to do.
If action A brings selective advantage, then those individuals who perform action A will over time live longer (or at least be more likely to live long enough to have offspring) than those who don't perform it, and thereby tend to procreate more and pass their predilection to perform action A on to more offspring. If one of those individuals happens to have a mutation that causes pleasure when action A is performed, then that individual will perform it more often... rewind and restart.
As the original post shows, even good things can be overdone. If the individual performs action A to the exclusion of other actions necessary to survival, it loses its selective advantage. Nature kills losers.