In order to understand this you need to see 'German' as an ethnicity and not a nationality. The Germanic people have long shared a cultural history dating back to before the Romans, and there were occasional calls for unity. The Holy Roman Empire did this to an extent, although not in reality. In the 19th Century, Otto Von Bismark envisioned a 'Greater Germany' under which all Germans would live, however he only ever achieved the creation of a 'lesser Germany' as Austria was never incorporated and was instead part of its own Empire (Austro-Hungaria). It is for this reason that Adolf Hitler annexed Austria, and it is also for the same reason that many Austrians supported the annexation: That is, the Austria people considered themselves to be ethnic Germans. As a result, I think, of the outcome of World War two, and the the fact that 'German' became seen more as a nationality than an ethnicity, the people of Austria have since distanced themselves from this association, although they still share a common language and much in terms of culture.