Hassan's normal smile is ironic for two reasons. He was born smiling, but even that was not enough for his mother not to abandon him. When he comes out of the surgery that Baba pays for, he smiles despite his swollen, raw face and thanks Baba and Amir, but even that smile and gratitude are not enough to win Amir's true loyalty and friendship.
Secondly, Amir actually states at the end of Chapter Five that
"by the following winter, [Hassan's post-surgery jagged line] was only a faint scar. Which was ironic. Because that was the winter that Hassan stopped smiling" (47).
Hosseini is a mastermind at ending his chapters or scenes in such an ironic and suspenseful manner. In the above example, he causes the reader to want to read more to find out what happens to Hassan to cause him to lose his new-found perfect smile.
Baba paid for Hassan's surgery to correct his hairlip in Khaled Hosseini's novel The Kite Runner. Like his father, Ali, Hassan was born witha physical deformity; Ali's crippling polio, which caused a pronounced limp, was never corrected. Baba decided to present Hassan with the surgery as a birthday present, which caused Amir's immature jealousness over his father's attention to Hassan to grow further. Sadly, once the harelip was corrected, Hassan rarely showed the great smile that was once commonly seen; with his relationship with Amir deteriorating following the rape by Assef, Hassan rarely finds reason to flash his smile, even though now it would be one not marred with his facial imperfection.