Most people like to imagine that normal life is happy and that other states are abnormalities that need explanation. This is a pre-Darwinian view of psychology. We were not designed for happiness. Neither were we designed for unhappiness. Happiness is not a goal left unaccomplished by some bungling designer, it is an aspect of a behavioural regulation mechanism shaped by natural selection. The utter mindlessness of natural selection is terribly hard to grasp and even harder to accept. Natural selection gradually sifts variations in DNA sequences. Sequences that create phenotypes with a less-than-average reproductive success are displaced in the gene pool by those that give increased success. This process results in organisms that tend to want to stay alive, get resources, have sex, and take care of children. But these are not the goals of natural selection. Natural selection has no goals: it just mindlessly shapes mechanisms, including our capacities for happiness and unhappiness, that tend to lead to behavior that maximizes fitness. Happiness and unhappiness are not ends; they are means. They are aspects of mechanisms that influence us to act in the interests of our genes.
Randolph Nesse, ‘Natural Selection and the Elusiveness of Happiness’, in Felicia A. Huppert, Nick Baylis and Barry Keverne (eds.), The Science of Well-Being, Oxford, 2005, p. 10