This is my headcanon, written as my FB status the other day:
Aang: "Hard work, bravery, and training. That's hard to pass up. Sounds like I belong with Team Valor." Katara: "Discovering things and learning how to use new techniques is my forte. Team Mystic it is." Sokka: "We all know my instincts are my greatest strength. You two are on your own, but I'm going yellow." ...... <silence> ...... <sudden frantic tapping on touchscreen> Zuko: "WHERE IS TEAM HONOR???"