When Americans think of kites they visualize beautiful sunny days, nice winds and a beautiful kite, or a kite of a favorite childhood character, or perhaps a sophisiticated "cool" kite, but they don't think about flying a kite as a cut-throat competitive thing to do on a breezy afternoon.
In this novel, we learn that in Afganistan, kite flying is a competitive sport and that there is a yearly contest. The object of the contest is to not just fly your kite and avoid having it get caught in a tree, but to avoid having the kite string cut by a another's kite string. The object of the game is cut each other's kites down in flight and remain the last kite flying. It takes a lot of skill to just fly a kite, but to become offensive in your handling of it brings the whole thing to a new level. Amir's drive to win the kite contest and make his dad pay attention to him -- be proud of him -- is so huge that he stands by while Hassan is brutally attacked. If he had interferred in the attack he could have been hurt himself, but he could have also lost the winning kite to Assef, the bully. Amir's actions in this moment define him for the entire book, and the kite becomes a symbol of friendship, connection to the past, and redemption as the novel goes forward.