In The Kite Runner, how does Amir feel when he realizes he and Baba are alike, and how is it both a positive and negative realization?
When Amir finds out that Baba lied about Hassan, he realizes that he—Amir—and Baba were more alike than he'd known.
Amir was shocked about hearing of his father's secret, and became very irate, even storming out on his longtime friend Rahim Khan, who was dying. Amir refers to an old cliche a writing teacher "would have scoffed at: like father, like son" (226).
Although Hosseini never clearly mentions Amir's emotions, we can sort of guess based on his thoughts; for example, he mentions how "Baba and I were more alike than I'd ever known. We had both betrayed the people who would have given their lives for us" (226). I suppose the emotion here would be remorse, perhaps. It's kind of difficult to find the right word, but he basically comes to the realization that he can't be necessarily angry at his father since he betrayed a close friend too (actually, a brother!).
As far as a positive realization, I think Amir knows that he has to find Sohrab, but he still wishes he didn't have to do it and just live on in his "oblivion". He has to find him, as he says, to "atone not just for my sins but for Baba's too". He is reluctant, but yet driven.
In addition to the emotions described above, the reader might also infer that Amir feels somewhat relieved.
Amir has always looked up to his father, and in his eyes Baba is as close to being perfect as is humanly possible. On the other hand, throughout his childhood he had always felt inadequate and like he was falling short of his father's expectations. While these feelings perhaps lose their edge as Amir matures and becomes more self-confident, they still persist into his adulthood, particularly because of his betrayal of Hassan, which Amir blames on his lack of courage and strength, the qualities Baba values.
When Amir finds out that Baba in fact lived a lie for a good part of his life, and also betrayed his loved ones, he must feel somewhat vindicated. If Baba was not perfect, there is less pressure on Amir to be perfect himself, which he thinks is expected of him even after his father's death.