In order to answer this question, you have to know something about the Treaty of Versailles, which was presented to the German leaders on May 7, 1919. The treaty was created by Allied powers with little input from the Germans. It had 15 parts and 440 articles. Here are some of the main points.
This treaty forced the Germans to give back land - Belgium, Czechoslovakia, and Poland. The Germans also had to accept Article 231, which is also known as the "War Guilt Clause," which stated that the Germans started World War I. This was a humiliating point for the Germans. Practically speaking, the Germans had to pay serious reparations. The German army was also limited to 100,000 men. Furthermore, no air force was allowed, and only Naval vessels under 100,000 tons were permitted.
What compounded things was that Germany experienced inflation, which led to hyper-inflation. This greatly hurt German citizens; it wiped out the middle class and unemployment skyrocketed, which caused social unrest. All of this led to a new right-wing nationalism, which ultimately led to the rise of Hitler.