I'm gonna sound like an extreme narcissist but I spent the last ten minutes just taking photos of my face with my phone using the back camera. In the first picture I thought I was fugly as hell. But as I took more and more pictures and looked at more and more of them, I began to see myself improve in looks up until the point I thought I actually looked good... I think this is because I was sort of desensitized to the little flaws on my face after looking at it so many times and started to see myself the way other people would see me. I was no longer paying attention to only the flaws on my face but also other features because the flaws no longer stood out to me. Nobody else would look at you and only see yours flaws. Our flaws only stand out to ourselves because we all tend to be quite self-critical. Such is the nature of human beings. But I guess this is one of the reasons why people can improve so much. However, for some people this "instinct" is too extreme.
This is why we're actually better looking than we think we are or just better than we think we are in general.