When a body is starving for sugar, ketones take over and help breakdown the next best source of energy: stored fat. The body produces ketones during starvation or fasting and also during long bouts of physical training. When your muscles are sore after strenuous exercise - ketones cause the pain.
Khaled Hosseini published The Kite Runner in 2003. By the end of 2005, it was a bestseller in the United States. It seemed readers couldn't get enough Hosseini's story about the troubled friendship between two Afghan boys.
One answer might be that Hassan haunts Amir and is, in one way or another, the subject of Amir's thoughts for pretty much his whole life. It's also worth mentioning that at the end of the novel, Amir does become a kite runner. The book ends with Amir running a kite for Sohrab. It seems to be another step on his path to redemption that he does for Hassan's son what Hassan used to do for him.