I think that the experience of Hassan's betrayal at the hands of Amir in the most critical of moments is something that helps to shape their relationship. In the short term, it drives a potential wedge between them, causing separation to be evident. In the long term, it causes Amir to believe in redemption, in the hope of becoming "good again." It is this experience that ends up furthering their relationship as it drives Amir to return to Afghanistan and end up adopting Sohrab. Experiences also help to shape relationships in the fact that Amir has to end up viewing Assef as the personification of evil, something or someone that Amir himself seeks to not become. The relationship is forged out of the experiences of seeing him rape Hassan in the alley and then as a member of the Taliban upon Amir's return to Afghanistan. In this, experiences forge relationships. Finally, Sohrab comes to trust Amir out of the experience of being saved by him, seeing how much Amir loves the boy, and then the experience of Amir serving as Sohrab's kite runner at the end of the story. These experiences help to shape and form the relationship between the boy and Amir at the conclusion of the narrative.