The US Army has done a lot of studies in this area of late, and found that a certain percentage of the population is somewhat resistant to PTSD after a traumatic event, but there are upwards of 20% or so that are pre-disposed to it as well (http://www.ptsd.va.gov/professional/newsletters/research-qua...). It's sad that it has taken centuries of warfare for people to wake up to the realization of this and finally just start learning how to identify and treat it.
A good friend of mine, an Iraq war combat veteran, runs a blog and non-profit on this subject as well at http://www.combatveteranswithptsd.com
If only we could do away with the stigma associated with it now which is falsely based much upon wild dramatizations in movies and elsewhere of eccentric, crazy or violent veterans (http://flcourier.com/2013/07/03/post-traumatic-stress-disord...).