It's all brain chemicals

There are all kinds of factors that can influence levels of certain chemicals in the brain. The one I am going to focus on is the neurotransmitter, serotonin.

Levels of serotonin have been found to correlate with social heirarchies in vervet monkeys, as this study found. The higher the blood serotonin levels, the more dominant the monkey was over the other monkeys.

But what influences serotonin levels? Stress, lack of sleep, lack of exercise, poor nutrition, etc and possibly a number of other genetic and environmental interactions. If unmodulated, reduced serotonin can lead to mental problems like depression.

These are all fairly predictable from the viewpoint of evolutionary biology.