When trauma occurs, the more “primitive” parts of our brain related to survival take over. These primitive parts do not have the ability to process on a “higher” level (I.e., logic, reasoning, rationalization) and as a result, trauma is encoded in the body on an emotional [but not rational] level. Therefore, trauma survivors tend to react based on emotion and not reasoning.
Considering this, most trauma therapy works by re-experiencing the trauma in a safe, controlled environment while applying logic and rationality to the traumatic experience. Over time, this will help decrease the emotional intensity of the trauma and reduce the resulting PTSD symptoms.