In The Kite Runner, what lessons did Amir learn about life and humanity by the end of the book?
Amir learns through experience that life is, as playwright Thornton Wilder once wrote, "a terrible, wonderful thing." For instance, Amir suffers as he watches his father die of cancer, but his life is deeply enriched by his marriage and Soraya's abiding love for him. Throughout his life, Amir has observed human behavior, including his own. He has witnessed courage and cowardice, as well as selfishness and sacrifice. In returning to Afghanistan, he experienced the tyranny of corrupt power as exercised by the Taliban and the desperation of the powerless under their rule.
Amir's most profound insight, however, concerns the basic nature of human beings, including himself. Amir learns that people are flawed creatures--even Baba, whom Amir had always viewed as being beyond reproach. In discovering that Hassan, too, was Baba's son, Amir gains an understanding of his father as a man, not an icon. This knowledge makes it easier for Amir to accept himself, including his own flaws.
The most affirming truth that Amir realizes, the one that enables him to achieve self-respect, is that human beings can be redeemed through courage, integrity, endurance, sacrifice, and wisdom. In returning to Afghanistan to save Sohrab, Amir saves himself.
Amir learns a great deal about himself over the course of the novel, but I think one of the most important things he learns is about the nature of love and family. Amir's pivotal decision not to help Hassan at the beginning of the book is motivated by jealousy and pride; he chooses not to intervene in part because, as the son of a powerful man, he thinks that is what a person of his rank ought to do. When he discovers that Hassan is actually his half brother, Amir's understanding of his father and of his relationship with him changes. This emotional development is paralleled by Amir's growing cultural awareness as he deals with life as an immigrant in the U.S. and grows into a man. By the time he realizes that he must return to Afghanistan to find Sohrab, I think Amir has made another, more fundamental discovery: the bonds of love and family are far stronger than any cultural or socioeconomic ties.