Keeps us on the chase (McMahon)

Although natural selection has conditioned us to experience pleasure in activities that contribute to our survival, it disposes us to quickly adapt to them and then to strive for a little more. In this view, enduring satisfaction or permanent contentment would not be conducive to survival. It is in our interest—and so in our genes—always to be slightly wanting, restlessly searching for further satisfaction. A bit of anxiety keeps us on our guard against danger, and a bit of unfulfilled desire keeps us on the chase, ever eager to ensure our survival and that of our kin.

Darrin M. McMahon, Happiness: A History