Hassan lives in a hut while Amir lives in one of the nicest houses in the area. Hassan being the servant while Amir is the richer. This is what shapes who they are, and how they see each other when they're older.
Even though they live different lives Hassan feels they are best friends while Amir only sees him as a servant to keep him company. Hassan does all of Amir's biddings misinterpreting it as playing.
Another part of how different their lives are is how their dads see them. Hassan's dad loving, while Amir's dad resents him. Knowing this, Amir thinks the annual kite fighting tournament will change that.