

Yangchen gave a long speech that excused the Avatar from the Air quest to let all love and connection go.

Kuruk fell in love with Ummi, who had brown eyes, so maybe mixed. Of course that means the Zuko must have died just before Tenzin was born, which is unlikely since it's been confirmed that he's not dead. It also was pretty much stated that Toph was destined to be Aang's earthbending teacher so she would be the reincarnation of Roku's waterbending teacher and that leaves Zuko as the reincarnation of his earthbending teacher. In "The Guru" it is implied that Katara is the reincarnation an Air Nomad (whoever formally taught Roku). So just who Katara, Toph, and Zuko reincarnated from? My guess is that they follow the same Avatar cycle. The old masters were great teachers, but not for the Avatar: Pakku treated Aang like any other student (which sounds fair until you realize that Aang does need special attention since he has to master the elements much faster than other students do), Yu didn't even know that Aang was the Avatar until Toph's parents sent him to track her down, and Jeong Jeong couldn't teach him because of the circumstances.

He starts out learning from some old master (Pakku, Yu, Jeong Jeong) but most, if not all, of what he learns come from a kid closer to his age (Katara, Toph, Zuko) Why? The reason for everything in this show: Because Destiny Says So. The Avatar's teachers are reincarnated with him/herSomething that always got to me was that Aang could only learn from certain people.
