The Kite Runner

I'm on page 310 now.
In my opinion, the most three themes that run through the novel are racial problem, lies and the way to be good. The most important thing that goes through the novel is the lies. Amir lived in a whole world of lies. He didn't know Hassan was his half-brother because his dad and Rahim had told a lie to him and they didn't even tell Ali about that. And the most hilarious thing is Amir always heard this sentence from his dad, "The only sin that mattered was theft. When you tell a lie, you steal a man’s right to the truth." If Amir had known that Hassan was his brother, even just half-brother, he wouldn’t have done those mean things to Hassan, like put his gift watch under Hassan’s pillow and made Hassan and Ali away. The whole story would’ve been changed if Amir had known that, but it’s the part that attracted me a lot. And the story indicated that sometimes there is something more important than the truth. It’s also so dramatic that I cannot guess what is going on next. Racial problem is also a huge issue that caused a lot of pities. It might because Ali was a Harazan that made Sahib and Rahim kept that as a secret. And those concepts were irrigated to people’s mind when they were young. The Taliban head, Assef, was also influenced by that. He was hurt by Russian soldier and made him to a Taliban guy with a cruel soul. The last point of this book is the way to be good. Indeed, there’s always a way to be good. Agha Sahib did a lot of things to make up a recover for his sin. And Amir was trapped by his sin too. He was going to compensate for his sin too, taking back Sohrab and raised him up. Those three themes are going through all over the book and combined together tightly.
Question: Why do you think Amir changed his mind and compensated for his sin?