Where does national identity come from?

National identity is one's identity or sense of belonging to one state or to one nation. It is the sense of a nation as a cohesive whole, as represented by distinctive traditions, culture, language, and politics. National identity may refer to the subjective feeling one shares with a group of people about a nation, regardless of one's legal citizenship status. National identity is viewed in psychological terms as "an awareness of difference", a "feeling and recognition of 'we' and 'they'".