I recently was thinking on the similar topic after reading an article about scientific myths most people believe. For example, one of such myths is that humans are genetically predefined and can't be changed through life. It's clear that there are a lot of social, cultural and gender factors impacts on our nature, but still there is such a belief.

So my conclusion was that our reasoning is very restricted. We tend to simplify our thoughts, our memories. And all this stuff is highly dependant on emotions we feel. That's why we like to make our memories brighter than they are, make them more romantic and ideal. Everything in the past was better, etc. That's why we remember facts which are easier to remember and close to our beliefs, sometimes no matter are they true or not.