Most people I know feel like aliens sometimes. As an adult, I think I feel it for different reasons than when I was younger: now, it seems to have to do mostly with the fact that I have made a lot of non-mainstream choices and interact mostly with equally non-mainstream people, so when I encounter what I'm sure are "normal" people, my major reaction is "are you for real?"

As a kid, I think it had to do with lacking some social clues, which led to being boggled by the social weirdness of adolescence: at the end of 5th grade, I got on with everyone in my class, and suddenly and inexplicably, in 6th grade, I was unpopular! :)