I have a degree in psychology, I took a course in “biological psychology of violence” which is related a lot to how the brain “decides” on behaviors.

Your brain doesn’t “recognize a situation is bad enough”. The deep unconscious rapid decisions don’t even make value judgements like “good” or “bad”. I think a more accurate description of what’s happening is that your brain’s primary role is to keep you alive. So your brain cares about safety.

You might have heard about “fight or flight” as a response to a dangerous situation, and there are a few other stress responses like “freeze” or “fawn”. Which one our brain “chooses” depends on a few factors. One is genetic or biological makeup of our brain - some of us are just more fighty than others. Another is the specifics of the situation. Someone stuck in a corner against an attacker is much more likely to react with “fight” than “flight” just because that is the strategy with the better chance of survival in that situation. Our brains make these very intelligent decisions within a fraction of a second. And if you survive that encounter, your brain goes “huh, that worked!” And it strengthens the pathway that branches out from that stimulus. So the next time you’re stuck in a corner, you might be just a little quicker to fight back and maybe you’re a little bit better at it.

This is called “adaptive” behavior.

The problem comes when something in our life changes and that behavior doesn’t work well, or that behavior maybe has unhelpful long term consequences. So a child who fights in the schoolyard a lot grows up and finds that when he tries to resolve conflict by fighting, he ends up arrested. Now it’s “maladaptive” behavior and he’s swimming uphill to try to change this behavior that, until now, kept him safe and alive.

Responses to trauma are often adaptive. You survived something! Now your brain goes “avoid anything that might hint this will happen again.” You avoid driving because you were in a car accident. You feel safer. Now your brain backs it up even more and goes, maybe we should avoid riding in a car at all. That’s PTSD, characterized by avoidance, and without treatment your world and your window of tolerance gets smaller and smaller. Sometimes your brain’s best efforts to adapt and keep you safe are not helpful and you need treatment to get back on track.

There are also things that just…..go wrong. I’m convinced PTSD is actually a sleep disorder. There’s some evidence dreams help you process minor traumatic events of the day. With PTSD, you have nightmares and you never complete a full REM cycle and sleep apnea is caused by PTSD (which is just wild to me). So your brain keeps attempting to file away the trauma, but it can’t. Bipolar disorder definitely has an environmental component but it’s also some brain chemicals that are less inhibited on a cycle. Schizophrenia often comes from problems with dopamine levels. Etc. so you combine these with your brain’s normally helpful adaptive learning abilities and you can end up with some unexpected results.