Why is Amir's scar important in his journey toward forgiveness in The Kite Runner?

On a symbolic level, the scar is important for a couple of
reasons.  After the fight and the surgery, Amir's scar resembles Hassan's hair lip
before his surgery.  In this light, it moves Amir closer to Hassan on the road to
forgiveness, almost as if he is physically resembling the form he wishes to embrace.  It
becomes important that Amir move as close to Hassan as possible.  Since he is seeking a
way "to become good again," to move closer to Hassan, who is now dead, is vitally
important.  The scar is one such move.  Another way is how the scar was obtained.  Amir
was taking action against Assef's cruelty, something he failed to do as a child.  It is
not lost that Amir failed to take action with Hassan, but is not missing the chance to
take action for Hassan's son.  When Amir laughs, it is a moment of clarity present, as
he feels peace, while being beaten to a pulp.  It for this reason that that scar is
critical on Amir's path to forgiveness and "to become good
again."