I think that his mother asks Changez to remove his beard out of an "old world" and traditional notion that when he is returning to America, he should do as much as he can to fit in and assimilate with life there. Changez recognizes this. His refusal of his mother's request is to assert the opposite. He wants to stand out. When he arrives in Pakistan, he is horrified at how he has changed, viewing everything as an "American," or an "outsider." Only after he has caught himself doing that can he recognize the beauty of this world that he once knew and a world that is still a part of him. For example when he lands in Pakistan, he views his house in the manner of an America. Over time, he is able to reconnect with it and in the process, recognize its intrinsic beauty. In much the same way, he does not want to assimilate into America or "being an American" anymore. His trip to Pakistan begins to illuminate him as to the path he must take. In this process, he decides to keep the beard as an assertion of his own identity as a person from Pakistan. He does not heed his mother's words because she is speaking from a point of view that Changez once embraced. Yet, things have changed in his mind. One of these realities is the idea that he no longer has to be anything but what he is. He perceives the keeping of the beard as an example of this idea.