Genes are important for understanding our behavior. Incredibly important - after all, they code for every protein pertinent to brain function, endocrinology, etc.

The frontal cortex is an incredibly interesting part of the brain - ours is proportionately bigger and/or more complex than in any other species.

Essentially, we humans live well enough and long enough, and are smart enough, to generate all sorts of stressful events purely in our heads.

The less it is possible that something can be, the more it must be.

Some Poor grad student pressing on the flanks of a hamster and out comes a doctorate on the other side

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 probably even the case that if you stoked up some Buddhist monks with tons of testosterone, they'd become wildly competitive as to who can do the most acts of random kindness.