Humans as a species are designed to have a short memory about certain things. It is partially why someone that has underwent the pain of child birth might be inclined to go through it again with another child. If we could truly remember certain experiences as they were, we would be less likely to do them again. It is a pretty cool biological mechanism we have going on.
Even good things we have a tendency to downplay. This seems by design as well. If we as species were truly content with what we have achieved, we might not continue to strive for improvement.