A couple days ago my parents told me I did certain things when I was eight or so that were really embarrassing. I found this weird because I don't think I would forgot something as bad as that. My moms explanation was "We all remember selectively".
I think that is really true, and kind of scary. We all forget things we did wrong that we don't want to remember. I wonder what else I don't remember.
If you look at it at not a personal but a societal scale, it's true too. People tend to talk less about wars they consider unjust, like the war in vietnam or Iraq, than wars that are considered just, like World War 2 or the union side of the Civil War (Although arguably, no war is just). And when you look at it from societies from a religious stand point, things get really interesting.
Take ancient Egypt for example, when I was in kindergarten in sunday school I learned that Egypt, it's many idols, and it's evil slavery were all unholy and terrible. However, in sixth grade (At a religious school) Egypt was referred too a great, industrious civilization. Each side seems to have forgotten about the other. Or Rome, for another example. I know priests who refuse to acknowledge Rome as a civilization because of Pontious Piolot, yet every time they vote they are entering into a Republic-style government that was partially developed by Rome. And those who praise Rome's government (Before it got all corrupt) forget that that government is the one that ordered Jesus executed.
In my opinion, selective memory works best in positive light, and although it is slightly deceiving, it is necessary to having a functioning history book. If we take all the crimes of even the greatest civilizations, they look terrible. Forgetting the crimes of individuals in thinking of a society is the best way to remember, in my opinion. Of course, turning someone into a hero (By putting them on the $20 bill for example) who did terrible things shouldn't be done.
Sorry if this post was a bit off, I am running out of ideas.