Why is it we do what we do?

I saw a note posted on the overpass support beams as I drove to work this morning.  I was just thinking about the same thing - why is it we do what we do?  It was probably some advertisement or something, but the timing was just too perfect.  So here I am sharing my thoughts and opinions on this age old question.
I have a friend who believes in astrology and feels that everyone acts a certain way based on their astrological sign.  I don't have a problem with that, but I don't think it's that simple.  First of all, astrology is not simple at all.  Astrology can be very complicated if you really want to use it as a predictor of human behavior. Our astrological sign (sun, moon or other) may give us a predisposition to do certain things or have certain preferences, but so do our genes.   Our upbringing and our life experiences help to mold us, too.
For example, I am a Sagittarius and therefore averse to commitment, astrologically speaking.  But I was raised by a single parent and I'm also a divorced mother of four, wouldn't that make me a little cautious in the relationship department?
I also believe in free will.  We may have a predisposition to do a certain thing, but in the end we decide what we want to do.  As a child, we didn't have many choices.  Our parents or the other adults around us made the choices and we had to live with the consequences.  Now, we're in the driver's seat.  It may not be easy or fun, but we have to make the hard or uncomfortable choices to have the life we want. 
A new season is about to begin, a time to reflect.  Do we like the place we find ourselves?  If so, great, keep doing what you're doing.  If not, it's time to ask ourselves what we should do differently to find ourselves in a different situation.  It may be time to do the hard or uncomfortable things.
Why is it we do what we do?  A combination of what we bring with us at birth and the experiences we have had since getting here.  I know I need to make some changes and some hard choices, but I feel that I am up to the challenge.