There can be many reasons for that and most have nothing to do with life choices or "mental models" whatever that means. * Trauma triggers and unprocessed/unco

There can be many reasons for that and most have nothing to do with life choices or "mental models" whatever that means. * Trauma triggers and unprocessed/unconscious trauma. You'd be surprised how common this is. People who grew up in neglectful or abusive homes tend to believe there's something wrong with them and that their childhood was fine. So when life feels like shit, like there's no point, and it's only suffering every day, that somehow other people manage and they don't, they see no light at the end of the tunnel. It must be their fault, and for some reason they can't "get over it" or "Let it go". * A person repressing stuff, which at some point reaches breaking point or saturation. * Combination of (unlucky) factors - e.g. winter/less sun, less people around for some reason, had to sacrifice exercise for extra work, (...). * Chemicals in the brain indeed, change from one day to another. And maybe the day they feel depressed is when they are seeing things more realistically and crash with that realization. I don't buy this idea that it's part of the human condition that we're bound to have depression sometimes and that it's chemical imbalance. If we look at human history of psychiatry you'll see how coldly we've been treating patients. You'd be horrified at how we treated mentally ill people in the past. Why would you think we're doing any different now, just in a more mODeRn way - "They're sad because their brain is wrong. Give them a pill."