Americans' proclivity for displaying the flag is a way of expressing unity in a country that doesn't have a common ethnicity, religion, or language.
Nationalism also facilitates integration in a country that has tons of immigration. My family is from Bangladesh, where group membership is determined by ethnicity. A white American could move to Bangladesh at a young age, speak the language fluently, marry into a Bangladeshi family, but he'd never be Bangladeshi. Here, you step off the plane, put that flag up in front of your house, and boom you're American.