In the beginning of the book, it explains what the lamb represents, Amir see's the lambs eyes, " it is a look that will haunt my dreams for weeks." (p.76) Amir imagines that the sheep knows what' happening to itself. That is why he watches the sacrifice every time. He thinks that "the animal sees that its imminent demise is for a higher purpose." (p.77) Amir is talking about the lamb because he is watching Hassan being raped right now, and is comparing them to each other.
As Hassan is being raped by Assef, Amir is there watching it happen. Amir was watching and then he stopped, but he needed to make a decision, he "could step into that alley, stand up for Hassan." (p.77) Amir wanted to help Hassan because he always stood up for Amir in the past. He could help Hassan, "or I could run. In the end, I ran. I ran because I was a coward." (p.77) Amir was scared that Assef would hurt him if he tried to stand up for Hassan, so he acted like he never knew it happened. Hassan was like the lamb in that very moment, he was vulnerable and couldn't do anything to get away.
Sohrab ended up in the hospital because he did something to himself because he didn't want to go back to the orphanage. Amir found a way for him to go back to America with him, so he went into the bathroom and, " suddenly I was on my knees screaming." (p.343) Amir was upset because Sohrab didn't get to hear the good news and he was hurt and now Amir had to take him to the hospital now. When Amir was waiting for Sohrab he about to take a nap and hoped that what happened in the bathroom was just a dream, "the left arm dangling over t he side of the tub, the blood soaked razor sitting on the toilet tank" (p.348) Amir didn't think Sohrab would do that to himself.
From Hassan being raped and Sohrab cutting himself, those were huge sacrifices. That symbolizes the lamb throughout the story. Sohrab turns into the lamb from his sacrifice. Amir changed into a better person because of what he went through. If you go through that much misery throughout life you will view the world differently and make sacrifices just as the lamb sacrifice in the novel.