[can16358p]

Probably evolutionary: our brains have evolved into hardcoding those memories to avoid getting into that bad situation again.

[derefr]

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.