Truth is, our lizard brains are always forgetting the good stuff that happened but remembering every detail of some random shameful event nobody, except you, cares about.
Or, to put that another way: of what practical use is a memory of being content? It has no informational content to feed to your predictive model of the world.
Memories of unexpectedly getting rewards are quite salient, of course, because those train you to get more rewards.