Oh, subjective obstinacy had advantages, Morris, when we were busy evolving into nature’s champion egotists. It led to human mastery over the planet … - David Brin

We are programmed to find sex pleasurable for one simple reason—because animals who mate have offspring. Those who do not mate have none. Traits that result in successful reproduction get reinforced and passed on. Evolution is that simple.
It is therefore useless to bemoan as evil the fact that men tend toward aggression. Among our ancestors, aggression often helped males have more offspring than their competitors. “Good” and “evil” had little to do with it.
That is, until we reached consciousness, at which point, good and evil became pertinent indeed! Behaviors which might be excusable in dumb beasts can seem perverted, criminal, when performed by thinking beings. Just because a trait is “natural” does not oblige us to keep it.

The lesson they took home with them was simple; it takes a full belly before a man or woman gives a tinker’s damn about anything as large as a planet.

What was it like, he wondered, to care about something so passionately? He suspected it made her somehow more alive than he was.