As it is said in the novel, he smiled because he thought, ” caught up in the symbolism … that someone had so visibly brought America to her knees.” Even though he didn’t really talk about the racism he experienced, it is sure he experienced it in his life in America. Of course, he wasn’t happy about how he was treated which eventually made him have some hatred towards America. Especially, he wasn’t happy about how his home, Pakistan was treated by America and how Americans treat themselves as the best in the world. Also, in his life, it can be shown that he didn’t have close friends who are white except Erica. At his work, his only friend was Wainwright who is not a white person. All people who he needs to be respectful of were white too such as Jim, I suspect his professors in Princeton too. This relationship with the American people led him to have frustration with him because no one can put them on their knees as they’re many and society in America is naturally white-dominated. However, when 9/11 happens, even those white people are brought to their knees by people from nearby his country which made Changez have some sympathy. Therefore, I think the frustration he built up in his life in America exploded when 9/11 and made him laugh. Basically, the cause was implicit racism. However, this event made him have more hatred as racism against his race became worse after 9/11. Before this, Changez hid such feelings towards this country and kept inside himself but after this, his frustration started to leak outside of him and became observable by others as Jim realized Changez isn’t happy at that time.