Aang. Why is he, for some, so insufferably cheerful? I say it's actually stems from his disappearance from the world for 100 years and the subsequent astonishment he has at facing his new world. He realizes that life is fragile and can be taken away and/or cut short at any point just like that, so he understands he needs to live life to the fullest and enjoy every day in this world as much as he possibly can. It's similar to the lesson Iroh learned from his son Lu Ten's death at the Walls of Ba Sing Se.
By my assuming this, I am not mostly discarding the effect of Aang's upbringing as a Air Nomad. I just think that the revelation of him being the Avatar at 12 combined with the genocidal attack on the Air Nomad would leave him questioning some of the more trivial teachings of the Air Nomads.
Do I make sense? Does this even count as a blog? Lemme know if you think I'm absolutely off my rocker!