"Some people seem to have a naturally higher baseline for happiness—one large-scale study of more than 2,000 twins suggested that around 50% of overall life satisfaction was due to genetics, 10% to external events, and 40% to individual activities."

In other words, genetics account for 50% and the environment the other half so that each of us can do a lot to make ourself 

happy even when our parents give us genes that tend to make us feel negative about everything.

I felt that this article is a nice analytical review of this important subject.