NavahoEverclear:
I doubt there's any 'exact' trigger.
Again, that's not a rare way to feel. When you're seventeen, you don't have as much perspective on people as you do when you're older. I would think - well I had the idea that everyone but me spent thursday through sunday partying, then the rest of the week was spent enjoying themselves with friends between classes. Nobody else was depressed ever and everyone but me had the most perfect family and boyfriend (girlfriend). Above all, nobody else could possibly identify with me or my interests.
Didn't help that my school was full of "trendy" people who definitely tried to convey that image, and very few of them could possibly identify with me or my interests.
Watching people up close will help. You realize that they have their own problems and uglinesses, and you stop comparing yourself to them in such an unhealthy way.
Thales:
All actions, even the "altruistic" ones, are based on selfishness.