Why do we always want what we can't have?

I was laying in bed last night wondering this question myself.  Why do we always want what we can't have. 

If its raining we want drought:  If we have drought we want rain. 

If its cold we want heat:  If its hot we want cold.

If we live up North we want to live in the South:  If we live in the South we want to live in the North.  



Goes to show you we are hard to please.  We always want what we can't have.  I guess I would say we get bored really easy and like lots of changes.