person who's born happy doesn't have to try hard

50 to 70 percent of the variation in people as to their level of happiness over time is genetically determined - according to James Maddux, professor and director of the clinical psychologist training program at George Mason University, in Fairfax, Va.

The above number, I believe, applies to most of human behavior -- 50-70% of human behavior are determined by DNA, the rest are determined by how much effort they to put into it.