The Holy Roman Empire was a bit more complex than just a German nation state, although it was majority German. It wasn't even called the Holy Roman Empire of the German Nation until 1512. The Italians and Dutch, because they had significant populations of Italians and Dutch living in their cities, were able to consolidate their own distinct national identities through an active middle-class intelligentsia. Prag on the other hand was mostly German, with Czechs living in the Bohemian countryside, hence why its inclusion into the German Federation was considered. It was only with the migration of Czechs into the cities with the industrial revolution that Bohemia lost any German identity. Point is, I personally consider the HRE to be better thought of with feudal, multi-ethnic and multi-cultural project.>>9875985
To be fair, what may be in the interests of one person may not be in the interests of another, one group of people in the interests of another group, one nation the interests of another. Couple that with the fact that geographic borders such as rivers of mountains are useful for national defence; and coal, metal ore and food are all obviously useful, and conflict between countries is to some extent inevitable.
Plus I would also argue that people generally enjoy being amongst people of the same or similar culture, as it gives them common values and rules around which to organise.