These two parts of the novel are parallel in many ways.  The most obvious is the use of the line "for you, a thousand times over."  We also see the chance for Amir to be good again in the introduction and his carrying out of that in the last chapter.

Both excerpts take us back to the tournament and the rape of Hassan.  Seeing Sorhab in a volatile state and Amir becoming nervous because of his memories simply enhance the uneasy feeling that we get when reading this book.

Basically, Housseini is opening and closing a circle: ending where he started.

This is a great question. One of the key episodes at the beginning of the novel, when Amir and Hassan win the Kite competition and Hassan captures the kite, is mirrored at the end of the novel but under radically different circumstances, as the roles have reversed entirely.

At the end of the novel, it is Sohrab who wins the competition and it is Amir who runs after the kite for him, and it is also Amir who finds himself saying what Hassan said to him all those years ago:

"For you, a thousand times over," I heard myself say.

At the beginning of the novel, this comment is indicative of the deep love and friendship that Hassan has for Amir, which tragically he goes on to betray so terribly as he does nothing to prevent Hassan from being raped. At the end of the novel, however, it is symbolically significant that Amir says this to Sohrab, indicating how far he has travelled during the course of the novel and how he has changed from a selfish, spoilt boy to a man who is brave enough to love and receive love in turn. Thus, his comment at the end of the novel shows how he has transformed himself. It is also significant to note that it is this episode that represents the first breakthrough into Sohrab's character and makes him smile.