Resilience
Why do some people develop post-traumatic stress disorder after a trauma and others remain symptom free? We are still trying to understand resilience, one characteristic of good mental health.
Researchers studying survivors of the September 11, 2001 World Trade Center attack have been surprised to find many of them to be free of PTSD symptoms. Resilience was less common among people who were actually in the buildings or in the immediate area, but the researchers report that even for "highly exposed individuals" the frequency of resilience never fell below one third.
Read more about resilience and the World Trade Center research.

Comments
I think this is an interesting perspective. I was diagnosed with PTSD in 1990. I can say from my perspective that I may have developed resilience as a result of the struggle with my symptoms of this disorder. The research on this subject peaks my interest as to whether or not resilience is an innate or learned trait.
That’s a good question. At the very extreme end I think that poor resilience can be either innate or learned (or both). Growing up in an extremely abusive home seems to make people more vulnerable to PTSD from later adult trauma. On the other hand, some people are resilient even though they come from such a home. Maybe that’s the innate variety.