Sohrab has been plagued with a number of traumatic experiences. He has lost his parents, he feels guilt due to the abuse he endured from the Taliban, and he feels guilty about hurting Assef. As Amir attempts to find a way to adopt Sohrab he shares with Sohrab that the easiest way may be to place Sohrab back into an orphanage and then adopt him. This, however, breaks a promise that Amir made to Sohrad earlier. Amir told him he would never have to go back to the orphanage. This is the straw that breaks the camels back.
In the book The Kite Runner many years before the confrontation between Amir and Assef over Sohrab, Assef had sexually raped Sohrab's father Hassan. When Amir goes to get the boy he is set up against Assef. Assef has Sohrab dressed like a girl. He also makes him dance and wear female jewelry.
One of the dominant indicators that Sohrab has been used for sexual play is the way that Assef holds him and takes the grapes in his mouth. He wants Amir to know that he violated Sohrab sexually.
Moving backwards to Sohrabs stay in the orphanage, the reader learned that the Taliban had taken boys and girls from there to be used for their own devices. Sohrab would have no trust in the idea of having to stay any where such as an orphanage. Thinking that he would be returned to one from which he could return to Assef's hands, he decides it would be better to end his life.