The exact numbers vary slightly from study to study, but most of the happiness research indicates that about half your happiness comes from the cortical lottery - some people are born with better pleasure receptors than others. This is the basis for one's "happiness set point" and surprisingly, life circumstances (good job / bad job; smart car / scruffy car) only account for about a ten percent fluctuation around one's "happiness set point" - the rest? It's all choice.
