The answer is simple. Unlike in France or Sweden or Cambodia, the citizens of our nation do not derive their national identity from a specific piece of land or a religion or an ethnic heritage, a race or even a language -- although it is possible to argue about the last one. To be an American is instead to subscribe to an idea, which comprises a philosophy of government, a means of organizing society and an economic system. This article was pure joy to read, inspiring and moving as well.
Here's an interesting piece that reaches the same conclusion -- i.e., the power of the American idea and the ruin that will surely come from abandoning it -- but from the vantage of a person who is not an American, but a German economist:
Obama's Misguided Approach: American Has Become Too European