This quote expresses the truth that Baba loves the idea of America but "living" in America gives Baba an ulcer. Hosseini includes this quote not only to portray the difficulty that many immigrants encounter as they try to adjust and be successful in America but also to give the reader a slightly autobiographical glimpse into his own family's early life in America.
Baba and Amir choose America to flee to because Baba sees it not only as the land of opportunity, but also because he admires America's rough, adventurous reputation. While in Afghanistan, Baba emulated America's Wild West with his car, bold attitude, and swagger. When he arrives in America, however, he finds that his sway in Afghanistan has disappeared. His name means nothing in his new country. Once known by all in Afghanistan, in America, Baba is forced to work long hours for a pittance and show identification in order to make minor purchases. He is out of his comfort zone and must worry about how he and his son will survive. Like many immigrants, Baba grew up with a glorified view of America but finds it difficult to make his dreams come true in actual America.
In regards to a second reason for including the quote, I believe that Hosseini wanted to pay homage to his own father in a sense. Like Amir, he fled his homeland with his father and family when Afghanistan was no longer safe. His father was a diplomat for Afghanistan and well respected. When they moved to America, he was forced to take whatever job he could and worked long hours for minimum wage to provide for his family. Hosseini himself loves the idea of America but stresses here that success does not come to Americans without hard work and some pain (i.e., the ulcer).