I feel that Amir is a huge jerk to those under him and continually teases Hassan "because he can". Hassan, being the nice but slightly naive and foolish person that he is, keeps running back to Amir and forgiving his wrongdoings.
Throughout the Kite Runner, Amir continually teases and takes advantage of Hassan. An example of this is when, at the beginning of the winter season in Kabul, Amir and Hassan have a conversation before they prepare for the kite running competition. In the conversation, Hassan asks Amir rhetorically, "Would I ever lie to you, Amir Agha?" Suddenly [Amir] decided to toy with him a little. "I don't know. Would you?" (Hosseini, 57). The conversation is continued and Hassan replies to Amir. "I'd sooner eat dirt," [Hassan] said with a look of indignation. "Really? You'd do that?" [Hassan] threw [Amir] a puzzled look. "Do what?" "Eat dirt if I told you to" (Hosseini, 57). Even Amir describes himself as "being cruel, like when [he'd] taunt him if he didn't know some big word" (Hosseini, 57). To Amir, "there was something fascinating - albeit in a sick way - about teasing Hassan. Kind of when [they] used to play insect torture. Except now, [Hassan] was the ant and [Amir] was holding the magnifying glass" (Hosseini, 57). Another example of not being a proper friend to Hassan was when he was being raped and Amir stood watching on the sidelines. "[Amir] stopped watching, turned away from the alley. Something warm was running down [his] wrist. [He] blinked, saw [he] was still biting down on [his] fist, hard enough to draw blood from the knuckles. [He] realized something else. [He] was weeping. From just around the corner, [he] could hear Assef's quick, rhythmic grunts. [He] had one last chance to make a decision. One final opportunity to decide who [he] was going to be. [He] could step into that alley, stand up for Hassan – the way he'd stood up for [him] all those times in the past – and accept whatever would happen to [him]. Or [he] could run. In the end, [he] ran" (Hosseini, 137). This passage really highlights Amir's value in Hassan as a friend. He says that Hassan has stood up to Amir so many times, but Amir ran like the coward that he is in Hassan's time of need.