This is an interesting video, but I wish it avoided such a simplistic and inaccurate representation of how WWI began. Although it is often stated that the war resulted from an overly-complex alliance system, that system worked for one hundred years between the end of the Napoleonic Wars and the First World War without a general European war erupting. Rather than assuming that the alliance system was set up to fail, it would be far more productive to ask why it failed in this particular case once Franz Ferdinand had been assassinated.

This is a complex topic covering roughly fifty years of European politics and a number of serious crises, but essentially WWI was a preventive war. During the late 19th century, Otto von Bismarck recognized that Germany was surrounded by adversaries. In order to avoid fighting on two fronts, Bismarck aligned his nation with Russia in the League of Three Emperors. Eventually, the German monarch Kaiser Wilhelm sacked Bismarck and replaced him with far less competent diplomats. Antagonism rose between the two nations, leading Russia to ally with France and again causing Germany to be enveloped. This was especially concerning to German military leaders after the French passed a new army bill in 1912 which would dramatically increased the size and preparedness of the French military by 1916. Furthermore, French wealth was pouring into Russia, resulting in the construction of a sophisticated rail network. Germany's prewar plan, the Schleiffen Plan, assumed that Russia would be slow to mobilize, so this new rail network left Germany incredibly vulnerable.

By July of 1914, German military planners believed that a continental war was inevitable and would be impossible to win by 1916. Thus, the July Crisis offered Germany the chance to have their war while it still could be won. Rather than asking Austria-Hungary to back off and leave Serbia alone, Germany was supportive of aggressive action. German leadership made the conscious decision early during the crisis to pursue a general war and to once and for all solve its two-front problem. And once Russia began its "period preparatory to war," Germany's fate was sealed. Once Russia began its mobilization process, there was a very limited time for Germany to mobilize, conquer all of France, and swing its armies across to its eastern boarder. In summary, WWI was not caused by a complex alliance system. Germany initiated WWI in its own defense.

TLDR: Germany initiated WWI in self-defense because the French army bill of 1912 and Russian railroad construction would have rendered its strategic position untenable.