What are three reasons why World War I led to World War II?
It was not so much World War I that helped pave the way for World War II as it was the post-World War I treatment of Germany, combined with effects of the Great Depression and the high levels of unemployment that resulted that created the atmosphere in which World War II was destined to break out.
1) When France and Great Britain, the victors in the First World War, imposed terms on the defeated Germany, they were not particularly prescient in anticipating the ramifications of those conditions. The terms under which hostilities ended were codified in the Treaty of Versailles, which required Germany to relinquish large tracts of territory on which resided large populations of German-speaking people; the payment of reparations to the victors, which made Germany's already battered economy even weaker; and prohibited Germany from rebuilding its armed forces beyond minimal levels deemed sufficient for self-defense. Particularly egregious from the psychological point of view, and exploited by the rising German politician Adolf Hitler, was an article in the treaty (231) that required Germany to accept all responsibility for the outbreak of the war -- a highly questionable assertion.
2) The Great Depression of 1929, the effects of which would be felt well into the 1930s, especially in Europe, facilitated the rise of Hitler, who exploited the German people's economic misery for political gain, enabling his political party's ascendency and his appointment to the chancellorship.
3) The efforts of European political leaders to appease Hitler fed the latter's appetite for territorial conquest. By hoping that Hitler's invasion of the Sudetenland, then a part of Czechoslavakia, would satiate the German dictator's appetite for conquest, a hope codified in the ill-fated Munich Agreement of September 1938, the British in particular helped to ensure that another world war would occur.
These are the main reasons for the series of developments that lead much of the world into another major war. World War I could only be considered a precursor to World War II if one looks at the post-war agreements that laid the foundation for further conflict.