Answer:
Militarism: The Arms Races, with war looming nations were desperate not to be outdone in any area of their military. Germany, France and Russia started to conscript large portions of their male population, this gave their countrymen military experience and a taste of war, suddenly everyone was ready and willing to fight. This conscription and arms build up meant that soon every European state had a huge army at its disposal, these armies were ruining their economies, they had to lose them soon but couldnt just let them go, they were propelled to war in order to destroy their enemies and give them the security they needed to allow disarmament. Germany also began building a large, modernised navy that directly threatened the British, who they were still attempting to secure as an ally at the time, and eventually forced them into the arms of the French, which leads us onto......
Alliances: without alliances WW1 would merely have been a short war between Serbia and Austria-Hungary, the alliance system dragged all of Europe into that insignificant conflict, first Germany and Russia and then the British and French a few days later. It was what made a small war a major world conflict.
Imperialism: Again Germany's search for both a colonial and Central European empire put a lot of pressure on the Triple Entente and made a war more likely. They constantly threatened Britiain in order to secure concessions of territory (particularly in Africa and the Pacific), again this only tightened Britain's links with France and added to the Alliance web.
Nationalism: This was what started the war and what allowed it to continue for so long. Previously wars had been very short and there had generally been at least a modicum of healthy respect for the enemy in many previous European conflicts. By WW1 the populations of Europe had been assured by their governments propoganda that their enemies were the lowest of the low. It was the hatred of the Germans that made the French want revenge for their occupation of Alsace Lorraine, it was the hatred of the Germans that allowed the British Army to have a sudden surge in recruit numbers in 1914 when it deployed to France and it was that same hatred that compelled the Allies to keep on fighting through the horrors of trench warfare
