A single person is a manageable entity, whom you can either make friends with or leave alone. But half of a married couple is not exactly a whole human being: if the marriage is successful it is something a little more than that; if unsuccessful, a little less. In either case, a fresh complication is added to the already intricate business of friendship: as Clem had once remarked, you might as well try to dance a tarantella with a Siamese twin.