These are amazing points, and a very interesting post. I have thought about this very often, as I am the only one of my two (younger) siblings to have developed PTSD. All three of us grew up in the same alcoholic household with a narcissist father.
I have been told that I have PTSD because I'm too sensitive, too intelligent, that I think too much, that I expect too much from people. My therapist has said that no one really knows why some people develop PTSD while others do not. Some soldiers--for example, my wife's grandfather who fought for the Soviet army during WWII--never develop a single symptom.
Stir Ling - I am the youngest of three and the only one who has PTSD. But my two siblings are in denial. While I did get the worst of it they got it bad as well. Our father was also narcissistic and that in and of itself took a toll. One that I would not even begin to understand until I became an adult - searching for understudying about why my life was not working. Unfortunately my siblings and my mother will not discuss our experiences while living as a family. That is so painful for me. I think we could make so much progress is we would work together , but that won't happen.