Why are journeys undertaken? What is the outcome of undertaking a journey?
Within the context of The Kite Runner, there are two significant journeys in the book, the first from Afghanistan to America, and the second, from America to Afghanistan. The first journey results in Amir running away from his problems, while the second results in his facing his problems, and to the degree possible, solving them.
Amir has betrayed Hassan, his friend and brother, in deep ways, for example, by not defending him against a bully and by allowing Hassan to be blamed for a theft. As Baba and Amir flee Afghanistan, that betrayal is unresolved and unatoned. Whether Amir realizes it or not, it colors everything that happens to him in America, a guilt he always carries with him. He tries to atone in many ways, by being a better son to Baba, through work, by respecting the traditions of his culture as he falls in love, by trying to do the right thing. However, all of this does not alleviate his guilt because the person whom he has wronged is not the person he is doing the right thing for at all.
So, his journey back is about atonement, about being "good again." Amir must travel to Afghanistan to try to right the wrongs he has done. Sadly, he cannot atone to Hassan, who is dead, but what he does do is seek out Hassan's son, who is in the hands of the Taliban and the bully Amir did not try to save Hassan from the first time. This is a kind of reenactment for Amir of the original betrayal, not quite a "do over," since Hassan is gone, but still, a way of righting the wrong to the family.
So, journeys can be about running away, and the message in that might be that one cannot ever runaway from oneself. But running toward atonement can be the basis for a journey, too, which is what Amir's second journey results in, atonement and a form of redemption. Putting things back to the way they should have been is seldom possible, but being "good again" is always possible, no matter what!