“Look, whatever else happiness is, it’s also some kind of chemical reaction. Your body making and experiencing a cocktail of hormones and other molecules in response to stimulus. Brain reward. A thing that feels good when you do it. We’ve had millions of years of evolution that gave a reproductive edge to people who experienced pleasure when something pro-survival happened. Those individuals did more of whatever made them happy, and if what they were doing more of gave them more and hardier offspring, then they passed this on.”
“Yes,” I said. “Sure. At some level, that’s true of all our emotions, I guess.”

“I don’t know about that,” she said. “I’m just talking about happiness. The thing is, doing stuff is pro-survival—seeking food, seeking mates protecting children, thinking up better ways to hide from predators...Sitting still and doing nothing is almost never pro-survival, because the rest of the world is running around, coming up with strategies to outbreed you, to outcompete you for food and territory...If you stay still, they’ll race past you.”
Source: Short fiction, The Man Who Sold The Moon (2014), p. 130