This is an opinion - not based on any factual evidence whatsoever!!! - I think the human brain is usually programmed to dull the memories of bad things, in order to protect itself. Therefore when we look back on events, any of the negative things that were going on get filtered out. I can remember spraining all the ligaments in my ankle last year, I know I was in agony, but i can't remember what it felt like, I just laugh now about how I was stuck in the middle of nowhere with 2 young children, no mobile phone, and no ability to walk. Ha ha ha we say, wasn't that a funny old day - when I think really really hard I CAN remember what it was really like, but it wouldn't do me any good to relive those feelings. I said above that the brain "usually" works this way - there are plenty of people whose brains seem to work the oposite way, and only remember the bad things.