Mystery: Something which is Foo behaves like a Foo, and something which is not Foo doesn't

Free Exchange | S.C. | America's fiscal union: Blithe acceptance

In a blogpost on Friday, Paul Krugman pointed out that income gaps between euro-members are no greater than income gaps between American states. The difference, he writes, is that

we think of ourselves as a nation, and blithely accept fiscal measures that routinely transfer large sums to the poorer states without even thinking of it as a regional issue

There's a good reason for that: we actually are a nation, and Europe is not.

Mrs SB7 and I have frequent "fiscal transfers" between ourselves. My unmarried friend Anne and her boyfriend Bob do not. Would anybody look at that situation and decide the difference is that Mrs SB7 and I "think of ourselves as a family"?