Avatar Themes - The Consequences of Our Actions.
Aang feels immense guilt for abandoning his position at the Southern Air Temple, but most importantly, abandoning Monk Gyatso, his longtime parental figure and closest friend. He ran away from the world after hearing that the monks were separating the two of them, and must now deal with the repercussions of his selfishness.
The themes of Avatar have taught us many things, one of them being that there are consequences for our actions. We must always consider how something we do will effect somebody else - even somebody we know and love. Aang will never know for certain what would’ve happened if he hadstayed… and because of this, his guilt will never fade.