I haven't done exhaustive research on just why PTSD occurs. I do know that there is some kind of physiological alteration in the brain. Perhaps it has something to do with how acutely a person reacts to things. In my case, it was long-term damage that was reinforced over a period of 20 years or so.

Nevertheless, in major, long-term cases there seems to be a "burned-in" response to certain stimuli. Why? I can mostly cope with my feelings. I just can't change the initial physiological response that happens in my brain whether I want it to or not. I can learn to shut down that response once it happens. But I have not yet figured out how to make it go away altogether.