I’m guessing this is more of a philosophical question and I’m more interested in your personal opinion. Factually, one could argue that it was the assassination of Franz Ferdinand, or that it was about colonies, stopping German mainland dominance, etc.
But for me it’s quite hard to grasp why WW1 had to happen. It’s hard to find any rational reason. It’s almost like a bunch of people with stiff hats and handlebar mustaches idolized war and in the end they let out an uncontrollable Monster that consumed human flesh by the millions. As if it was the climax of 19th century romanticism.
Like no one knew what they were going into and no one really knew what they were fighting for, only that they can’t lose.
In the end even the victors came out worse than if they had just lost 4 years earlier.
WW2 was basically the rematch of WW1. WW2 also had ideologies battling with each other. WW1 had no ideologies, just a pointless waste and suffering of millions of human lives.
What are your thoughts?
You can point to some widespread elements that contributed to the war. Social Darwinism, nationalism, militarism may have, to debatable degrees, made the leadership and population of Europe more accepting of the idea of a general conflict. There was likely also an lack of appreciation for the risk of a drawn out exhaustive conflict by some leaders and the general public. While there is some evidence (from quotes from Moltke and other military figures) for instance that the possibility for a long war wasn't totally disregarded, the more popular opinion was that a long war was impossible because no nation could afford it economically. This idea was put forward, perhaps most famously, in The Great Illusion by Norman Angell, and in the end I think that we'd have to agree he was largely right if only the actors in the drama had had the foresight to see.
At a lower level, though, each major player had their OWN specific reasons for accepting the war... things they wanted to get out of it.
Austria-Hungary wanted to punish Serbia for a track record of supporting separatist agitation and terrorism. Austria also strongly (and correctly) suspected, and had some but not enough proof, that elements of the Serb government were actively involved with the assassination of Franz Ferdinand. Austria also saw an opportunity to strengthen their influence in the Balkan region which might have enabled them (in their own excessively optimistic minds) to build an extended economic zone.
Serbian leaders knew that if they allowed a proper investigation, the Austrians would be able to justify overthrowing the Serb government and installing a puppet (because they knew they were busted). The only hope the Serb government had of retaining it's position was to fight it out and hope the Russians decided to help them and could beat the Austrians.
Russia (who had no treaty with Serbia, so don't "web of alliances" this situation) had a desperate need to defend their last remaining proxy in the Balkans and to restore their credibility in the region. After having been seen as letting down the Serbs in 1908 by not contesting the formal annexation of Bosnia by Austria, seen (by previously preferred proxy Bulgaria) as betraying Bulgaria in 1912/1913 in favor of the Serbs during the Balkan Wars, and seen by the Serbs as not supporting them in 1913 when they tried to annex Albania, there was certainly a track record of diplomatic "failure" and "weakness" growing. The Russians also had long term goals to acquire Constantinople and the Turkish Straits, which were so important to their economy, and a general war could offer at least a restoration of their Balkan influence and possibly the possession of the straits (if the Ottomans joined the Central Powers and if Russia won, of course).
Germany had this encirclement syndrome thing going on. They saw France building up for war with the new 3 year law, in theory leading to a 50% increase in peacetime army size and over time a similar increase in reserve manpower. They saw similar build ups in the Russian force, as well as vast improvements in Russia mobilization speeds that could nearly rival the Germans ability to deploy to the Polish front. Germany also knew that France had been unwilling to fight over Balkan issues in the past (1908, 1913) and that Russia had always backed down in the past. In the opinion of German leadership, if Russia didn't back down this time too then that meant France wanted a war. Germany probably overestimated the future military strength of France (there was a pretty good chance that the 3 year conscription law, which was very contentious when it was passed, wouldn't have lasted long) and definitely overestimated Russian military ability in 1914, but hindsight offers us these advantages.
France worried that they would be at risk of losing the Russian alliance after not supporting Russia in 1908 and 1913 during those Balkan crises. France did not have to support Russia in 1914, yet they chose to do so for their own reasons. The record of diplomatic communication and activities of the French ambassador Paleologue show that French officials went so far as to encourage Russia to escalate the situation rather than attempt to calm the crisis. There were also many in France that longed to avenge the defeat of 1871 and the loss of Alsace and Lorraine, but sometimes that influence gets overplayed. That sort of thinking did exist, however, in the public and in government.
So all the major players had reasons that, in June/July 1914, they thought were of strategic importance either politically, militarily, or both. They all had opportunities the step away from the war, but each decided the potential gains were worth the risks that war entailed.