Probably nothing to do with what we all think. Nothing about strength or weakness, how we were raised, etc....... there are a lot of things in life where the best answer is, "I don't know."
Too many variables that can mask this issue. What may seem strong is really weak, if one wants to use those terms. Like going into a battle, who gets hit, who gets KIA, who does not get a scratch when everyone else does. It may be nice, or maybe it is a human need to know and to especially to know be for hand. The toughest person may turn into a weak mouse at the first crack of gun fire, the person that everyone labeled a weak mouse, stands up to be the toughest person in the company........ why? Isn't that something we want to know, but will never know why.
PTSD as it was explained to me is when normal people are exposed to non-normal traumatic event (something like that). It becomes a issue later, because it has not been dealt with earlier. Right there is two variables, dealing with it, how ever one does closer to event can prevent or diminish the effects on the person later. Trying to deny what happen, being strong and ignoring what has happen and how it effected you, denial, etc............. will only make the situation more difficult later. It is the later, where the "P" comes into the meaning of the term, "Post".
The trauma needs to be dealt with sooner than later, that is what PTSD is about. Dealing with it later it becomes Post Traumatic Stress disorder.