why does Rahim Khal tell Amir about Homaira?

It was a little bit of therapy for Amir's tortured soul. Rahim Khan found him and told him a story. He had almost married a Hazara woman, but his family was outraged at the proposition and sent her and her family out of town. Then Rahim Khan told Amir that he could confide in him, but Amir could not bring himself to tell his friend what he had done. Rahim Khan gave him his present, a notebook for his stories. Then they hurried back to the party to watch the fireworks. In one flash of light, Amir saw Hassan serving drinks to Assef and Wali. He saw Assef playfully punch Hassan in the chest before, to his relief, the light faded.