Amir laughs when Assef beats him because the experience is cathartic for the beating he should have taken when he was a child.

When Amir and Assef were young, Amir refused to intervene as his loyal friend Hassan was sexually assaulted by Assef. Just before turning away from the scene, Amir recognized that he had "one last chance to make a decision" and "decide who [he] was going to be." Instead of intervening on behalf of Hassan, who had proven himself a loyal and courageous friend, Amir ran away, leaving Hassan to suffer. Amir carries tremendous guilt about his cowardly actions into his adulthood.

In chapter twenty-two, Amir confronts Assef as adults. Hassan has died, and Amir has come to rescue his son, Sohrab, from Assef's cruelty. Assef agrees to transfer the boy to Amir and then casually adds, "I didn't say you could take him for free." Assef challenges Amir to a fight to fulfill the "unfinished business" from the boys' childhood when Amir escaped Assef's threats.

Amir endures a severe beating, made worse by the brass knuckles Assef wears. As various bones in his body snap, Amir begins laughing. Through his pain, Amir has a realization:

What was so funny was that, for the first time since the winter of 1975, I felt at peace. I laughed because I saw that, in some hidden book in a corner of my mind, I'd even been looking forward to this.

The brutal beating Amir endures relieves the emotional anguish he has felt since that fateful day when he was a boy. He laughs because the beating ironically "heals" the agony of constant guilt he has carried for years.