The whole country thing is ludicrous. All countries are fake divisions, often created long ago, which are treated as if God given. They create huge problems and fix very little.
Humanity has always gravitated towards building societies centred around the concepts that modern-day nation states also embody. Our natural drive to band together and our tendency to be creating new fake problems all the time is certainly an interesting pairing at times.
Being part of only a single group isn't really our thing though. Without something external to focus on we quickly tend go down this path of surprisingly efficient self-destructive behaviour that often involves lots of heavy weaponry. We are chaos incarnate :)