How is the relationship between Amir and Hassan emphasized in chapter 8?

As they drove along in the car, one friend's twin daughters recounted Amir's victory at the kite-fighting tournament. At this, Amir's carsickness overwhelmed him and he vomited. As they aired out the van on the roadside, Amir saw Hassan's bloodied pants in his head.

Finally, they reached Kaka Homayoun's house in Jalalabad. Even though Amir finally had the intimacy with Baba he had wanted all his life, his guilt made him feel emptier than ever. As Amir, Baba, and everyone else slept in the same room, Amir confessed to the darkness, "I watched Hassan get raped." No one heard him. He realized that he was the monster in Hassan's dream and had dragged Hassan to the bottom of the lake. That night, Amir's insomnia began. We see the rape of Hassan will haunt Amir for the rest of his life.