Why can't I remember simple things sometimes?

It's simple really, each human has a unique brain. Everyone says that your whole life flashes before your eyes when you die but that's not completely accurate. If our brains kept every memory held inside, there would be hardly any room for anything else. But our brains are smart, as we go through life, our minds get rid of the memories that we don't need. We can only remember the times in our lives when a moment leaves some kind of mark on us whether that mark is physical, emotional, or mental.