I've been reading about the Second Boer War, another war that Britain thought would be over very quickly, but dragged out for 3 years or so, so why did they think WW1 would be over so quickly? Obviously it's difficult to compare the Boer War and WW1, because of the different environment and the whole Guerrilla Phase of the Boer War that took up a lot of it, but surely it would cause some doubt for a quick war between great powers?
Well, I can only explain for the German Side. Basically most Wars in Germanys recent history went quite fast (Austro-Prussian War, Franco-Prussian War etc.) so many Soldiers and Citizens of the Reich thought that the war would be over quickly since Germany's Army funding and popularity was really high and it also was one of the strongest Armys in Europe.
Besides having such a strong Army, most Germans (including Diplomats and Statesmen) though that Germany was quite superior to other nations. There is the term of the "Politik der freien Hand" which basically just meant that Germany could have alliances with any Nation (even France!).
Also you see in the 1800's most wars were over quite quickly (except the Napoleonic Wars of course) and thus having a strong Army made the german people think the war would be over at Christmas since France would be defeated quite quickly ( as in 1870/1871 ) and then Russia would also fall quite quickly because of the combined might of Austria and Germany which would basically end the war since the UK would be quite in a bad position if both France and Russia would have fallen.
One of the major miscalculations that military leaders made in the run up to WWI was that modern technology would give the tactical advantage to attacking forces. This miscalculation led many military leaders to believe that the war would consist largely of a series of quick offensives that would result in the quick capture of the enemy capital. This false reasoning was largely based on observations in the Russo-Japanese War that ended about ten years before the outbreak of WWI. European observers in this war noted the particular effectiveness of bayonet charges. Many attributed the Japanese victory to superior martial virtue, essentially claiming that the Japanese won because they were more aggressive. As a result, mainstream military thought believed that the most aggressive force, armed with modern weaponry, would win a war. Many assumed that this development of aggressive tactics would result in a brief war. However, the Russo-Japanese War turned out to be very different than WWI. As it turned out, the rapid fire capability of the modern machine gun that was employed in WWI gave an enormous tactical advantage to a defending force. It was easy to mow down an entire regiment of charging infantry or cavalry with a few machine guns, and so the war became a stalemate.