My current theory on "why do we think that way when we're young" is a disconnection in logic between the older perspective and younger perspective. You don't think you're invulnerable when you're young you simply just don't think about what happens next. When your brain matures and you get older you realize you're going to be around a much longer time than you ever imagined. Your pre-frontal cortex gets a stronger voice and says "watch out you'll die in a second doing that!"

An adults perspective is they young think they will never get hurt. A youths perspective never thinks about getting hurt in the first place.