(oh my god, finally a question where I can contribute)

Well that depends on what you mean with a "unified Germany". Austria itself never was part of germany, with the exception of the years 1938-1945. However, I'm not sure if you could say that there was ever a feeling of "superiority". But let's run through Austrian history really quickly to have a look at how it unfolded in parallel to germany:

The whole of what we today would call "central europe" emerged from a diplomatic mess of small principalities in the middle ages. Both "Austria" and "Germany" never really existed how one would imagine them until the 19th century. What today is Austria was part of the Hapsburg Empire, later called Austria-Hungary. The Hapsburgs - Originally a family from what we today would identify as switzerland - gained direct control over most of the southern german-speaking territories throughout the 14th Century, and would continue to hold or even expand their empire throughout the next 500 years. People in "Austria" probably identified themselves as germans tough.

( But maybe you are talking about the Holy Roman Empire? It - at some points - included all of the german speaking territories and is sometimes considered a proto-germany. That view is often misleading tough, because the HRE was a very loose conglomerate of states, that did not even have something like a military allience with each other. It's hard to identify this as a "german" union of principalities ).

So, when we look at when Germany was unified, we have to look later, at a time when the HRE had allready disbanded. In the 19th Century, after the napoleonic wars had completely ripped through this loose conglomerat of principalities and small states, it became clear that a unification of Germany had to happen at some time. Diplomats were considering 2 possibilities of how this could happen, termed the "Deutsche Frage" , the "German Question":

• "Die Großdeutsche Lösung" ("The great-germany solution"): This model saw most of what the old Holy roman empire covered as one unified state, under the leadership of the Hapsburg. It would create a strong Hapsburg-Empire in the heart of Europe. Throughout the first half of the 19th Century, this model was the internationally prefered idea. This would also make "Germany" a, for the most part catholic state
• "Die Kleindeutsche Lösung" ("the small-germany solution"): This is the opposite Idea. The Hapsburgs are to be left out of the unification of the german principalities, instead it should be prussia that takes a leading role in the new germany. This would also make "Germany" a protestant state, since most regions north of bavaria were, and still are, protestant.

In 1815, after the congress of vienna , the "Deutscher Bund" (German Alliance) was founded - today often seen as modern germanys direct predecessor - but it included all of the regions controlled by the hapsburgs. The great-germany solution seemed to be the direction in which things were moving. This changed in the 1860's when the prussian Ministerpräsident Otto von Bismarck became a driving force in the forthbringing of the "small-germany" solution. Prussia and the hapsburg Empire had a number of small conflicts in this time, resulting the battle of Königgrätz in 1866, which Prussia won. After this victory, Bismarck declared the Hapsburgs kicked out of the "Deutsche Bund" (the german Alliance), therefore making way for the "small-germany-solution". And indeed, in 1871 the German Empire was founded, without any of the Hapsburg Regions in it.

Okay so now we know why Germany didn't join Austria. But why didn't Austria join Germany afterwards? Well, up until the first world war, the Hapsburg Empire was still alive and kicking. We are talking about an Empire similar in size to the german empire here, and altough they were allies in the first world war, the rivalry between the two was way to big to ever consider a unification.

So, the first world war ends, the Hapsburg Empire falls and get's divided into many small parts. "The rest is Austria" is what is murmured the french President said after the victors were done making plans for the division of what used to be an empire at the core of europe. This new "Austria" becomes a republic, with ambitions to join Germany, this time as a unification of two democratic states. Still, after all this time under Hapsburg rule, the people of Austria think of themselves as Germans. But - the peace contracts of the first world war did not allow for such a unification. Too big was the fear of germany becoming strong enough again to crush through europe.

So, let's skip the next 20 years and look at 1938, the year of the "Anschluss", the unification of - now nazi-controlled - Germany with - now austro-fascist controlled - Austria. It is to this day an ideological question wether or not this unification was one wanted by the austrian people, but the answer is probably yes. yes, Austrians were glad to be part of the German "Reich".

Except, the Allies never got that part of the message. After the war, Austria was declared "the first victim of National-Sozialsm", and was seperatly managed as an occupied territory. But why? Why not treat austria as a part of germany? Here, the Sovjet-Union comes into play. A popular left-wing politian of Austria, Karl Renner, survived the war. Immediatly after the Sovjets came marching into Austria, he contacted the Sovjets and asked for the right to re-establish austrian government. Why exactly the Sovjets actually gave him that right, instead of installing a communist as would be their normal mode of function is hard to say.[ Edit: also see /u/commiespaceinvaders addition to this further down!]

So Austria is re-established as a seperate country. By now, "Austria" has existed as a small, independant nation for about a generation. In the 50's , when Allied occupation ended(55), something like an "Austrian Identity" has started to sprout. People under the age of 50 cannot remember a time under the hapsburg empire, only a time in the first austrian republic, and then the years under nazi-rule, which are understandably not something you really want to talk about. Since 1955, hardly anybody ever raised their voice in favour of a unification with Germany. After all, the last time this happend, it resulted in one of the worst crimes of human history.

Sources:

"Geschichte Österreichs, Kultur-Gesellschaft-Politik," Karl Vocelka (Sadly in German)

"Das politische System Österreichs", Hrsg: Heinz Fischer (Also, sadly in German. Sorry)

One of the key problems was also that the Austrian Empire was very ethnically diverse, with a lot of balkanic nations under its rule. During the 19th century, when the idea of a united germany began to gain approval, the austrians wanted to be included but with all of their population, a lot of which couldn't be considered german, and the other german states, including Prussia, Austria's rival and principal german opponent, wanted a more "nation-state" kind of country.

Source : Histoire diplomatique de 1648 a 1919 - Jacques Droz

Germany had a couple different opportunities for unification in the modern era. One resulted in unification and four did not.

1848- At the Frankfurt Assembly a "Greater Germany" was discussed but was ultimately rejected because of the difficulties of getting two powerful states, Austria & Prussia, to unite in a way that might benefit one over the other.

1919- The Big Four dictating the Peace of Versailles could have united Germany & Austria under the principles of self-determination and national unity. However, fear of German power through such unification led to a provision of the treaty actually prohibiting such a union.

1938- Germany pushed for unification with Austria and marched in before a plebiscite could be held. It is debatable whether or not Austrians opposed or preferred Anschluss.

1945- One might also say that Austria could have unified with Germany following WWII, but the manner of Allied occupation made this impossible.

1989- Austria could have unified with the reunited Germany's following their unification, but this was unlikely due to continued political divergence AND likely opposition from other European powers to a further strengthening of Germany.

Hence, it is debatable whether or not they themselves opposed such a union or whether, instead, it was simply politically expedient NOT to unite during the times that unification was possible. Nevertheless, the differences between Austria and Germany are primarily based on religious differences beginning in the 1500s. Austria is Catholic, as are a few other southern German areas, and north Germany is Lutheran. This led to political differences and social differences as the cultures diverged from one another following the Reformation and Religious Wars.

One of the biggest problems of the First Republic (1918-1938) was a absence of faith in the survivability of this new state. It was even proclaimed "Deutsch-Österreich" (German-Austria) in the beginning. Austria even sought unification with Germany. But as others mentioned that was forbidden by the Allies.

The new Austrian gouvernment proclaimed itself "independent" on 29th of April 1945, so before the Second World War was officially over. Right after the war was over, there was a tendency in Austria to attribute most of the horrific actions during the Nazi regime to the NSDAP or on the ethnic Germans in a broader sense. This dívision between "Austrians" and "Germans" was a way to shift blame. The role of Austria as the first victim of Nationalsocialism (as stated by the Allies in the Moscow declaration of October 30th 1943) further helped in this.

As mentioned earlier there was a strong case to distance itself from Germany and focus on "Austrian culture". This explains the rise of "Heimatfilme", basically family movies that concentrated on the beautiful nature and romantic fun in the mountains as well as a re-emerging fondess of the "peacful,dignified" days before the First World War under the benevolent Kaiser.

And sorry /u/Kugelfang52, there was simply no way of a Austrian-German merger in 1989. After over 40 years of independence and 1 of 3 worldwide UN seats in Vienna, and understanding as a neutral country between the two Cold War blocks