system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do distressing events or trauma lead to PTSD and other disorders?

When someone experiences a distressing event, what goes on inside their mind to increase their likelihood of developing a mental illness? Does it depend on how each individual reflects on and deals with it? Can somebody witness such events and not be effected in the same way or process it differently?
gpt
In a very natural very human way. (It's a little more complex than this but it's the gist)

If you are in a war zone for example, where your life is constantly in danger. Your body naturally reacts to protect you - this means highened awareness of surroundings, your body is more tense because it wants to be ready to run away or fight at any moment. 

The thing is, once your life/safety has been threatened (or when your body perceives a threat), your body often stays ready to fight or run away even when the danger is gone. 

Once your are in safety and your body is still acting like it's in danger, some problematic things can happen. You might have problems functioning "normally". Like you might be unable to trust enough to date or hold a job. Other people who haven't shared your experience won't understand why you act the way you do. They may think youre crazy since the threat isn't there anymore. Also, when your defenses are on all the time, physically it can very exhausting. Basically it's like high stress 24/7 - just unsustainable. 

Treatments for PTSD often involve teaching someone's brain and body that a) their reaction is normal but also b) the threat is gone.