Strictly speaking, Article 231 of the Treaty of Versailles never actually says Germany was 'guilty' of anything:

The Allied and Associated Governments affirm and Germany accepts the responsibility of Germany and her allies for causing all the loss and damage to which the Allied and Associated Governments and their nationals have been subjected as a consequence of the war imposed upon them by the aggression of Germany and her allies

Now, the difference between responsibility and guilt may seem like one of semantics, but it IS important. Think of it this way: If I hit you with my car and killed you, but was found to have been under the influence, I would be held responsible for what I did, but there would be mitigating circumstances. I didn't make a sober, pre-meditated decision to run you down! The purpose and aim of Article 231 was NOT to assert guilt for starting the war, but to assess responsibility for damages, and the reparations that would be owed for those damages.

Further more, the Article was not unique to Versailles; it was applied mutatis mutandis to Trianon, St. Germaine, Neuilly and Sevres. NO ONE ELSE complained about it, save Germany. Moreover, it states that Germany and it's Allies 'imposed' war on the Allies. Austria-Hungary DID open hostilities by shelling Belgrade; Germany DID open it's war effort by invading Belgium and Luxembourg; Turkey DID enter the war, forming an alliance with Germany in August, then allowing Goeben and Breslau into Constantinople, violating the neutrality of the straits, and then ENTERED the war with a Port Arthur-type action against the Russian Black Sea Fleet, followed by an offensive into the Caucasus; Bulgaria DID enter the war by invading Serbia alongside Germany and Austria-Hungary.

Now, I, and I'd say most historians today, recognize that it is far too simplistic to JUST blame Germany. However, I will say that it is the general consensus today that decision makers in Berlin and Vienna were working towards a war shortly after the shots in Sarajevo were fired, and little would be done to divert them from this course.

The issue of German 'War Guilt' became more pronounced in the 1920s, as the German government sought to undermine the Treaty of Versailles by taking Article 231, merely meant to lay a legal framework for reparations, and making it the CENTER OF THE TREATY! Disprove Article 231, the ENTIRE TREATY FALLS!

Holger Herwig has written an EXCELLENT article, called Clio Deceived: patriotic self-censorship in Germany after the Great War; it gives a detailed account of the 'war guilt section' of the foreign office which doctored and 'revised' documents to sanitize Germany's pre-war and July Crisis policies, essentially creating the idea of the 'Slither into War', that became the orthodoxy in Great War Historiography until Fritz Fischer's Griff nach der Weltmacht ('Germany's Grab for World Power') was published in 1961.

Some time back, there was a similar question of why Germany is always portrayed as prime evil in WW1, with some excellent responses which might answer your question.

When France and Britain forms some kind of alliance after centuries of war, then clearly something was happening in Europe that frightened them both. The Entente Cordiale didn't happen in a vacuum.

I won't say much more beyond that, I'll let the more informed make a clearer case.

In the grand scheme of things, it's hard to blame any one European Power for the beginning of the war. Yes, Austria-Hungary officially pulled the trigger with its invasion of Serbia, but then again, it wasn't the only country that wanted to settle some old scores with an adversary or enemy. Europe was riddled with nationalism, militarism, and alliances at this time. Germany was an extremely industrliazed and militarized nation, with one of the most powerful land armies in Europe (if not the world), and one of the largest industries in Europe. France remained thoroughly bitter about their loss in the Franco-Prussian War of 1870-1871, and sought to undermine Germany where it could. As soon as Kaiser Wilhelm ruined Bismarcks diplomatic work, such as the German alliance with Russia, France immediately allied with Russia. This antagonized Germany only further, as it enveloped Germany on two sides by two powers potentially hostile to it.

Russia herself was falling acutely behind in technological and industrial power, it had neither the industry to keep up with the industrial giants of central and western europe, nor the technological basis for a fully modernized army. Nonetheless, Russia sought to keep the European Powers down, Tsar Nicholas II in particular trying to get other European powers to sign an agreement to stop advancing technologically. Probably more importantly, Russia was seen as a defender of the Slavic people, and therefore showed willing to intervene on the side of any Slavic country, particularly Serbia. Austria-Hungary was a great power but secretly a heap of incompetence. They have had a series of disagreements with Serbia for a long time now, an animosity comparable to the animosity between France and Germany. The assassination of the Archduke was, in effect, a perfect excuse for the Austrians to unleash their pent up issues with the Serbians once and for all. Britain was perhaps the least eager for a war in Europe, and almost remained neutral if it were not for the Germany invasion through Belgium. Even so, Britain believed in the Balance of Power much like Bismarck, and it's hard to say that even if Germany didn't violate Belgiums neutrality, if it would stand idle and let Germany crush France.

In the end, Germany gets the blame for the war simply because France and Britain won the war and Germany did not. The reasoning they used was the "Blank Cheque" that Germany supposedly gave to Austria-Hungary prior to its invasion of Serbia. They saw it as Germany telling Austria that they can go to war, and Germany will support them no matter what. Since it was in France's interest in keeping Germany down, and keeping Austria down was not an issue considering the collapse of Austria-Hungary, imposing the Guilt Clause on Germany allowed Britain and France to impose the more harsher aspects of the Treaty of Versailles. In contrast, if Germany and Austria won the war, then the blame could have been thrown on Serbian aggression, and British and French intervention.

In reality, the cause of WW1 is hard to pinpoint even with the assassination of the Archduke. The atmosphere and willingness to go to war at this time (the romance of war still remaining a very real attitude) gives you an impression that was inevitable, meaning that no single power, even Germany, could probably have been blamed for the war.