Why are the boys (Amir and Hassan) and their fathers so affected by the gift Baba gives to Hassan?

On Hassan's birthday, Baba summoned him to the house as usual to collect his present. To Hassan, Amir, and Ali's shock, Baba had hired a plastic surgeon to correct Hassan's harelip. Amir was jealous that Baba was giving Hassan such special attention. The surgery went well and Hassan could finally smile an unbroken smile. Ironically, Amir explains, it was soon after that Hassan stopped smiling for good.