The good feeling of serotonin is a great motivator for mammals. Serotonin paves the neural pathways that wire a mammal to expect more in ways that worked before. Cortisol wires a mammal to expect harm from things that triggered it before. Alas, a mammal can't easily avoid stronger members of its herd or pack or troop because isolated individuals are quickly picked off by predators. It just keeps scanning for ways to feel good and avoid feeling bad.
Mammals who got more food and mates had more surviving offspring, which is why we've inherited a brain that rewards us with good feelings when we're socially dominant. "I'm not like this," you may say, but if you filled a room with people who said that, they would soon compete over who is more compassionate.
Monkeys make complex social calculations without a big cortex. All it takes is neurochemicals managed by neural pathways built from experience. Humans make social calculations with a cortex that analyzes lots of data about our social world. Words get our attention, so we don't see how neurochemicals focus our attention on certain pieces of information. We marshal facts to support a course of action that helps us feel good and avoid feeling bad. The big cortex serves the needs of the inner monkey.