In short, Germany was close to being defeated, and America wanted to be part of the post war carve up (when territory was divided). The US was supplying war materials to the Allies and could not do so while maintaining complete neutrality. There was also the sinking of the British ship Lusitania which angered the US. Also, the Zimmerman telegram.
The United States entered World War I for numerous reasons. A German victory would have upset the established balance of power. Should England and her fleet fall into the hands of Germany, the United States would have been in extreme peril to further German expansion. Americans were also upset by the sinking of the Lusitania and the Sussex, both carrying US citizens, by German submarines. In his campaign of 1916, President Wilson had promised Americans that he would maintain freedom of the seas. This eventually led to direct US involvement in the war in order to stop German subs, which were ignoring the US's position of neutrality, from attacking US ships.
