Why does Amir “enjoy” being beaten by Assef? How is this a turning point for Amir?

I think the fight and beating that Amir took was a cathartic experience for Amir. He had carried the guilt of not helping his friend Hassan for most of his life. The beating he takes from Assef is a release from so much of that guilt. This beating should have happened in Amir's childhood. At least now he can take the beating for Hassan's son and feel a small sense of atonement for his cowardice as a child.