Primates are super smart and organized just enough to devote their free time to being miserable to each other and stressing each other out.

If you're a gazelle, you don't have a very complex emotional life, despite being a social species. But primates are just smart enough that they can think their bodies into working differently. It's not until you get to primates that you get things that look like depression.

The stress response is incredibly ancient evolutionarily. Fish, birds and reptiles secrete the same stress hormones we do, yet their metabolism doesn't get messed up the way it does in people and other primates.

Robert Sapolsky Quotes