First Responders face dangerous and traumatic events on a daily basis. There is no way to know or predict which event could affect a first responder negatively enough to cause post-traumatic stress disorder.
Unfortunately, in a culture of bravery and pride, asking for help can be seen as weakness and many do not seek the help they need. They can be left to deal with PTSD on their own and revisit difficult and negative emotions over and over again. First Responders can feel like they are trapped in a painful past.