There is a lot of evidence that Happiness is much like IQ in that you are born with a basic peak and while it can be dragged down there is little that can rise it. For the most part, outside of extraordinary circumstances it's fairly constant over your lifetime.
PS: And yes this means there are probably people in the middle of a 20 year prison sentence that are on average happier than you are.
According to studies cited by Sonjy Lyubomirsky (http://www.faculty.ucr.edu/~sonja/ ) in her Book "The how of happiness" (a happiness book from a scientific view), the genetic predisposition is about 50%. 10% is your environment, and the rest is what you do with your life.
So, there is room for improvement for most people, although not as much as some happiness-gurus may like them to think.
PS: And yes this means there are probably people in the middle of a 20 year prison sentence that are on average happier than you are.