As it was explained to me, experiencing a traumatic incident doesn't immediately mean one will have issues with PTSD. Whether or not our brain can process the trauma does. If it can't, we get "stuck", and so relive the trauma, never getting past it. How we relive it is individual. Not having nightmares, not having flashbacks, doesn't mean you DON'T have PTSD. Only a trained therapist can help figure out that thorny subject. Having it, isn't a sign of weakness. It only means, whether due to biology, hereditary factors, past traumas, or other unknowns, you are stuck; can't process what happened.