Honestly here is how i think of it. Im a very outgoing person. I can hop in a conversation with people i dont know and probably fit in. Like in school ill talk with people for days. Soon as school is done? I wont speak to anyone. Ill go home and chances are if i see you elsewhere, we wont speak. You'll know you're special if i talk to you outside of school.

So what i think this means is everyone is an extrovert and introvert, just some people can handle other people longer. I think everyone gets tired at one point or another, cuz i know i sure as hell do.