Almost always, genes are about potentials and vulnerabilities rather than about determinism.

If I had to define a major depression in a single sentence, I would describe it as a "genetic/neurochemical disorder requiring a strong environmental trigger whose characteristic manifestation is an inability to appreciate sunsets.

We live well enough to have the luxury to get ourselves sick with purely social, psychological stress.

On an incredibly simplistic level, you can think of depression as occurring when your cortex thinks an abstract thought and manages to convince the rest of the brain that this is as real as a physical stressor.

It's great to have a buff frontal cortex to do that harder thing - for example, help a person in need rather buy some useless, shiny gee-gaw.

The gigantic challenge is the magnitude of the individual differences in the optimal set point for "good stress." For one person, it's doing something risky with your bishop in a chess game; for someone else, it's becoming a mercenary in Yemen.

Importantly, rather than promoting aggression, testosterone promotes whatever is needed to maintain status when challenged.