Probably the most important lesson from the book is that there is no wrong time to speak up. Amir's entire life is defined by his lack of action in protecting Hassan, even though he understands that if their situations were reversed, Hassan would not hesitate to help. Amir does not learn until much later, when he rescues Sohrab from Assef, that regardless of the outcome, he should have been willing to help a friend in need. Amir's experiences with Sohrab show him that there is more to life than self; without Amir's willingness to give Rahim Khan one last favor, Sohrab would have been abused for the rest of his life.
Another lesson is explained explicitly by Amir in the narration, near the end of the book. He explains that in America, the ending of a story is the most important secret; one should never spoil the end. In Afghanistan, the ending is important because people want to know what happens; it is considered polite to reveal the ending, not to conceal it.
If someone were to ask me today whether the story of Hassan, Sohrab, and me ends with happiness, I wouldn't know what to say.
After all, life is not a Hindi movie. Zendagi migzara, Afghans like to say: Life goes on, unmindful of beginning, end, kamyab, nah-kam, crisis or catharsis, moving forward like a slow, dusty caravan of kochis.
(Hosseini, The Kite Runner, Google Books)
This lesson is simply acceptance; life goes on. Despite Amir's efforts to make a better life for himself and his family, he cannot stave off death or pain; he is simply a player in a larger world. Instead, he learns to enjoy the present, to focus on the here-and-now instead of the future. He cannot know the end, but he can affect the present, as he does when he mirror's Hassan's enthusiasm for kite running for Hassan's son Sohrab: "For you, a thousand times over."
I learned that there isn't an end in life. Everything worse you do will come back to you, but you can solve everything in life.
The worrld is full of problems.