Citizenship (100% legal concept, sometimes referred to, in the States, as "nationality", in other languages "nationality" often refers to ethnicity), race (100% biological concept) and ethnicity/nationality/etc. (essentially, the community you grow up in) are 3 different things. Citizenship is bestowed upon you by birth, and are generally only bestowed upon you by other people, you can only choose to give one up, but can't choose your citizenship. Race is your genetics, you cannot choose it. Ethnicity can, to some degree, be chosen. Ancestry, as the OP refers to it, to some degree defines ethnicity. Virtually everyone you'll talk to in the States is an American citizen, and almost nobody will think of race when you ask (mostly because it's only consequential biologically, i.e., to a doctor). However, ancestry, i.e., what country their ancestors are from, is something people seem to care about. I am not 100% sure why. But, to summarize - I think people speak of ethnicity simply because it's the only thing you can, to some degree, choose, and as a consequence - there is a certain amount of pride (by association) that people are taught to have in association with their heritage. I disagree with this position (I think it's stupid for me to be proud of something I did not do), but I think most people find it OK to be proud OF someone other than themselves. (I.e., "proud to be an American").