Why was Germany angry that they had land taken away from them?

These lands were taken from Germany after the Treaty of Versailles. They included Alsace-Lorraine between Germany and France, Schleswig-Holstein between Germany and Denmark, and a good deal of land on the eastern border with Poland. Through the Treaty, Germany also temporarily lost control of the Saar Valley, an important industrial region. Germans were angry because they viewed this as an insult to their national honor. It is important to realize that World War I ended without Allied troops entering Germany. So the armistice was a shock to many Germans, and the Treaty was viewed as a capitulation or a "stab in the back" on the part of the new government. The land cessions were mostly lands that Germany had annexed throughout the nineteenth century, and many contained thousands of ethnic Germans. So to lose these lands was a real slap in the face to the German nation. It also, especially the loss of Alsace-Lorraine, had strategic implications by opening the frontier to potential invasion. In the context of the other affronts to Germany that pervaded the Treaty of Versailles, losing all these lands felt like an insufferable indignity.