You know that traumas tend to be easier to remember that anything else? This might seem totally off-topic, but it's not.
Pain is a trauma, if enough strong (of course i never talked of the pain like when a rock falls on your feet, but a much greater, both mental and physical). This means that pain, or to be more precise, the memory of a pain, a particular pain (mostly mental, even if physical one isnt escluded) can remain in your mind so brightly that you cannot behave in the same way.
On the other hand, the brain (or our mind, to be more precise. The brain is just the physical shell of our mind, our physical shield) might, especially in a young stage of growth, erase, no, block a memory, until someday, for a strange twist of fate or for a particular event to happen, this memory is called back, brutally, most of the time bringing to an altered vision of events for some time, in the worst of the cases resulting in insanity.
Some times the brain/mind tries to block them, but cannot totally do it, leading you to think something in a way or another, for a trauma that you cannot recall, but that is.
In conclusion, i still think that Pain, or better, after reflecting about it for a small time, Traumas are the only thing that can change the nature not only of a man, but of any living being.