This is a good question, and really interesting. @Tesseract has a big part of it.
- If you are helpless during the traumatic event, chances are higher that you'll develop symptoms.
- If you have little or no social support after the traumatic event, chances are higher you'll develop symptoms.
Resilience, as
@Chava says, plays a role. Resilience is not something you decide to have, and if you don't have it, it's not a personality defect. Resilience includes social supports, but it's mainly developed in early childhood. If you're securely attached - you have an affectionate, responsive caregiver, you're comforted when you're in distress, you are touched - then you have the internal resources, literally, the brain chemistry and the capacity in your body systems to return to equilibrium after a stressful event. (If you suffered a big trauma, it may take a while, but you're more likely to eventually recover.) If you have abusive or neglectful caregivers, or you suffer a loss or other traumatic event in early childhood, a later traumatic event is much more likely to lead to symptoms because you have less resilience.
@anthony notes that there are people who come out of "perfect" families who engage in risky behavior. I'd argue that (a) they might not be as perfect as they appear, or (b) that there was some single traumatic event in early childhood (rather than ongoing abuse or neglect) that caused a psychic wound. There is also some current research about what's called epigenetic effects - those are traits that are actually acquired by a parent or grandparent from environmental factors, which are transmitted to you the child or grandchild. The biology of epigenetics is pretty clear, and there is some evidence of epigenetic tendencies toward post-traumatic symptoms in the children and grandchildren of, for example, Holocaust survivors. There's also evidence of epigenetic effects of trauma in at least one rat study. So you might have a perfect childhood, but you've got some brain chemistry that makes you more vulnerable because of trauma suffered by a parent or grandparent.
I should note that there is something called trauma first aid. Peter Levine discusses it in one of his books. In
In An Unspoken Voice, he relates his experience of being a pedestrian hit by a car and how he discharged the traumatic energy afterward, in the presence of a supportive witness. He was able to recover from the event and did not develop PTSD.