In evolutionary terms, individual organisms are optimized for reproducing and living long enough to give their offspring (more accurately, their genes) a good shot at survival. Richard Dawkins explains that in The Selfish Gene and other books. "Contentment," a human social construct, and classifying a memory as "traumatic" probably have little to do with what evolution has optimized us (or any organism) for. If we reproduce and pass on our genes, then slowly die a miserable and traumatic death, that makes no difference in terms of evolution.