Towards the end , the friendship has taken a big fall and they are at a horrible stage. Close to loosing each other Hosseni tells us what happened between the two. "Then he'd taken the pomegranate from my hand, crushed it against his forehead" and Amir says "get up, hit me!" This helps show how Hassan was with Amir and looking up to him he can never betray or hurt him he will do anything for him but now the friendship has ended. Loosing a good friend that was so loyal and good to him must've hurt dearly but because of all the guilt he had he couldn't do nothing but drive Hassan away. It says " I remembered the day on the hill I had pelted Hassan with pomegranates and tried to provoke him. " This showing that Amir remembered the day he and Hassan got in that big fight and as a result their friendship ended. Since Amir couldn't tell Hassan that he saw what he had saw and was so guilty he lost a good friend.
Later in the book when Amir goes back to Kabul as an adult he goes to the tree and realizes that the tree hasn't born fruit in many years, just as his friendship with Hassan had died and not born the fruit of friendship , the tree had died also.