I think you are confusing nation with state, which is an easy to mistake to make when the nation-state -- the idea of state tightly coupled to nations -- has been a norm for several centuries, but while nation-states might be the current norm, nations and states remain distinct concepts.
In the specific context of Germany, while its true that Germany has been a nation for quite a long time (long before it was a single state), but nations are just social identities, and both sides in the Cold War were quite active in actively using propaganda to build "us" vs. "them" identities, and its not that surprising that without strong organized active supporting for the old pan-German identity and with active efforts to create opposing identities to replace it over a couple of generations, that there is a gulf created which will take another couple of generations of active effort -- at least -- to erase.
Ok, since we're kind of nitpicking over the word "nation": what do you suppose was the Holy Roman Empire of German Nation?
I'm aware that you were referring to the idea of a nation as it was established around the time of the French revolution. However, "Germany" as some kind of coherent construct is not just a recent concept.
That is the distinction between nation and state - nation just means people sharing language, culture, history and so on but does not imply living together in one state. So even while Germans lived in two states while Germany was divided they still were one nation. And I just want to explain the difference, I don't want to make any statement whether this distinction is relevant or not for the parent comment.
