At first happiness might seem like just desserts for biological fitness (more accurately, the states that would have led to fitness in the environment in which we evolved). We are happier when we are healthy, well-fed, comfortable, safe, prosperous, knowledgeable, respected, non-celibate, and loved. Compared to their opposites, these objects of striving are conducive to reproduction. The function of happiness would be to mobilize the mind to seek the keys to Darwinian fitness. When we are unhappy, we work for the things that make us happy; when we are happy, we keep the status quo. The problem is, how much fitness is worth striving for?

How the Mind Works