The very brief answer is that Germany was much stronger than Austria-Hungary, so as soon as Germany got involved, it became the center of attention for everyone involved. If you look at Austria-Hungary's dismal record in the war, it ought to be pretty clear why nobody was particularly afraid of them acting alone.

For more information, this sub has a FAQ about the Great War: https://www.reddit.com/r/AskHistorians/wiki/wwi

I'm not an expert on WWI, but I know enough to hopefully help answer a little bit. A lot of what I know comes from Christopher Clark's excellent book "The Sleepwalkers", as well as Barbara Tuchman's "The Guns of August."

What is important to understand about WWI is that no major national leaders wanted to fight a war, but they were all terrified of losing a war. Many military theorists of the time thought that a swift offensive strike was the best way to win a war. Many people also felt that war was ultimately inevitable, and that when the inevitable war came, the nations to strike quickly and decisively would be the ones to win. This led to a climate where everyone's plan was to strike fast and strike hard before the enemy was ready. Military leaders across Europe felt it was crucial to be the first to mobilize and attack. It also led to a great deal of worry about the consequences of being the last one to mobilize.

Different nations also had concerns about what they perceived to be the current arc of European power. Unified Germany was a young nation at this time, and only beginning to realize its potential. This terrified France in particular, but also led to concern in Britain and everywhere else. There was also a growing idea that a fully modernized Russia would be overwhelmingly powerful, a fear that especially struck close to home for Germans. France organized its alliance with Russia for the purpose of reining in Germany, but certain people in France thought the alliance was unstable and it would be best to try and start a war with Germany before the alliance fell apart. In Germany, they felt their only chance for survival if war were to come was to quickly knock out France and then focus on defeating the (supposedly) formidable Russian military. Certain German planners felt it was best to do this sooner rather than later, before Russia's military became unstoppable.

This led to a climate where the logical step to any situation was to escalate rapidly. Nobody wanted to be the one caught unprepared for an attack.

When Austria went to war with Serbia over the Sarajevo assassination, the situation quickly deteriorated. The truth is that Germany did not go to war because they made a careful, measured choice to stand next to their Austrian ally. They struck quickly because they were afraid that if they didn't, they would be stuck in an unwinnable multi-front war. Their decision was also influenced by the thought that they preferred to fight now, with a guaranteed Austrian ally, than to fight some number of years down the line without Austria at their back.

So at least when they went to war, Germany's primary strategic goal was knocking out their enemy's armies, guaranteeing German security for the foreseeable future. Their motivation was driven by fear of the worst, not really a strong desire for the upsides of victory. Most major actors at the time had similar motives. As the war went on, things changed, and my understanding of later goals is not nearly as clear, but at the beginning the primary goal was the destruction of the enemy's capability to make war for the foreseeable future.

The war is viewed as being Germany vs the Allies for a few reasons. Even at the time, many people in Europe (somewhat incorrectly) viewed Austria as a puppet of Germany. They figured that Austrian policy and German policy were coming from the same place. Germany was also the aggressor in the west - it was Germany's attack that opened the western front of WWI, and it was Germany's violation of Belgian neutrality that finally sealed the deal on Britain's involvement (before that, Britain was still debating whether it would enter the war). Even though it was Austria's declaration of war that set the ball rolling, many people at the time understood that the course of action to follow came down to a rivalry between Germany and other major European powers, and saw it as the moment of reckoning they all knew was coming one way or the other.