To out-group-members, oxytocin makes you crappier - less cooperative and more preemptively aggressive. It's not the luv hormone. It's the in-group parochialism/xenophobia hormone.

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.

Hormones influencing the sensitivity of the person to environmental stimuli.

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

Brains distinguish between an Us and a Them in a fraction of a second. Subliminal processing of a Them activates the amygdala and insular cortex, brain regions that are all about fear, anxiety, aggression, and disgust.

Fossey, Fossey, you cranky difficult strong-arming self-destructive misanthrope, mediocre scientist, deceiver of earnest college students, probable cause of more deaths of the gorillas than if you had never set foot in Rwanda, Fossey, you pain-in-the-ass saint, I do not believe in prayers or souls, but I will pray for your soul, I will remember you for all of my days, in gratitude for that moment by the graves when all I felt was the pure, cleansing sadness of returning home and finding nothing but ghosts.

Most people who do a lot of exercise, particularly in the form of competitive athletics, have unneurotic, extraverted, optimistic personalities to begin with. (Marathon runners are exceptions to this.)