In person, I probably talk about this idea too often, but a favourite thought of mine was from a friend in grad school. He explained that we are often defined by the reaction we evoke in others, as much as our view of ourselves.
This may explain why it can be so difficult to attend reunions, etc. When you meet people who haven't seen you since you were 19, they react (in a subtle way) in accordance with your social position/personality/etc when you were 19. We are ultimately immersed in a soup of emotion, reactions, etc that challenge our hard-won sense of security and sense of self.
A friend of mine from The Island recently described a holiday function back in Dec 2008. As he put it, he was 'surrounded by A-listers' from high school. He did not enjoy the experience. He wasn't 'cool' back then and, understandably, didn't feel too 'cool' in the present (see above). He felt anxiety that he hadn't felt in years and years.
I just browsed some folks on the dreaded Facebook (oh you've done it too -- just peeking around, not 'friending' people) and somehow managed to trigger the same feeling, after stumbling on various crowds from high school and lower grades.
Seriously, my heart started racing! Though I have chosen a life of singleness, and routinely flaunt the absurdity of my life as comedy, I am quite comfortable with who I am, and most of my goals.
It is embarrassing to have been vaulted back into a more insecure time.