While Amir has always assumed Baba's habitual generosity would present itself on every birthday the boys celebrated, he finds himself oddly unsettled at the sort of present Hassan receives one year.
Accordingly, Baba hires Dr. Kumar, a plastic surgeon, to operate on Hassan's disfigured lip. Amir initially sympathizes with Hassan's predicament, as he remembers his own painful memories from his circumcision experience the year before. Amir's own conclusion is that doctors are never fully candid about the degree of pain one has to endure in medical procedures.
At the same time, Amir feels conflicted and jealous about the implications of Hassan's birthday gift. To Amir, Hassan has elicited undue regard and largess from Baba on account of a preexisting condition, and not because he has done anything particular to earn this unprecedented magnanimity on Baba's part.
I wished I too had some kind of scar that would beget Baba's sympathy. It wasn't fair; Hassan hasn't done anything to earn Papa's affections; he'd just been born with that stupid harelip.
Amir is hurt that his own struggles to earn Baba's regard has met with little success, while Hassan has seemingly earned Baba's solicitous attention effortlessly. In the previous chapter, you will have read that Amir's foray into short-story writing was met with dismissive indifference by Baba. It is this discrepancy in Baba's attitudes towards the two boys which fuel Amir's jealousy and insecurity.