Another reason why the U.S. entered the war was because of an intercepted German telegram, made by Arthur Zimmermann that was on its way to Mexico. The British intercepted his telegram. When translated, this telegram told Mexico to declare war on the U.S., and if so, Germany would reward Mexico with American land. Although this was not much of a concern for neither Mexico nor the U.S., it still made Americans even more aggravated.
A final reason why the U.S. entered World War I was because supposedly, it was "the war to end all wars." This encouraged Americans to join the army, and it also increased American patriotism towards the war. Looking at the war as being the last one ever also supported the idea of world peace.
In conclusion, because of the sinking of the Lusitania, Germany disobeying the Sussex Pledge, as well as the Zimmermann Telegram and patriotism, the U.S. was forced to leave neutrality and join the Allies in World War I against Germany and the Central Powers.