The stranger is an element of the group itself, not unlike the poor and sundry 'inner enemies'—an element whose membership within the group involves both being outside it and confronting it.
In the end, I've found that it doesn't really matter who you marry. If you like them at the beginning, you probably won't like them at the end. And if you start off hating them, there's always the chance you'll end up thinking they're all right.