I am reading Iron Kingdom, a history of Prussia, and was struck by how often it gained and lost territories. After every war (and Prussia fought a lot of wars), it won/lost chunks while also gaining/giving up chunks elsewhere. Silesia, Hanover, Saxony ... the various states that it absorbed/traded lands with are difficult to keep track of. Heck, "Prussia" was originally the term for land east of the original land, centered around Berlin, that the Hohenzollern monarchy ruled; after annexing it the term for that bit of new land somehow became applied to the country as a whole.

The Silesians, Hanoverians, Saxons, etc. all viewed themselves as different people from the Brandenburgs/Prussians and each other. As toyg said, ethnic-based nationalism—viewing themselves as "Germans" and thus deserving a single German nation—is a 19th-century phenomenon; further, the various German states (not excluding Prussia) viewed and suppressed such nationalism as subversive. Mutual cooperation nonetheless grew during the century, notably the customs union.