Affectively laden experiences, especially life-threatening ones, tend to remain firmly fixed in memory, immune to the ablative processes of common forgetting known as extinction. This has been noted in disabilities related to war experiences, when many persons suffer a prolonged set of symptoms. In my experience, war-related PTSD presents more understandably under the rubric of chronic impairment or injury, rather than a mental illness or a prolonged reactive state.
Many patients with PTSD complain that there is no benefit to attempts at extinction of these events by repetitive re-awakening of memories of the experiences. They often perceive the re-awakening of these experiences as a sort of re-injury.