What gives Hassan the ability to forgive Amir and Sanaubar? Is there a change in Hassan?

I think Hassan has always had forgiveness in him. Hassan forgave Amir about the rape insident long  before Amir could begin to forgive himself. Similarly Hassan has no hate in his heart for his mother's indiscretions. Hassan's mother, Sanaubar, came to find him. She collapsed at the gate of the house; when they carried her inside and removed her burqa, they discovered that the former beauty was malnourished, had no teeth, and had grotesque scars all over her face from being cut. Hassan ran out of the house and was gone for hours, but when he returned he accepted Sanaubar as his mother. She became healthy and a part of the family; she even delivered Farzana and Hassan's son.