why do we do what we do

I don't why I'm bothering to think of this now, but I might as well before I fall asleep and forget about it.  What is the reason behind our actions, our motivation to madness.  I'd like to say I can rationalize my actions but sometimes I end up in WTF moments that just leave me speechless.  I love how we'll do something we know fully well will wind us up in trouble.  That we fall in love with someone even though we know they'll just break your heart </3.  We a crazy creature that feeds off of the needs of others.  Social sociopaths that do what ever they want.  And yet at the end of the day, when all has been said and done... we manage to function and not kill everyone.  That's a very good thing :)