I think the book For the Common Defense: A Military History of the United States of America said it best on page 346 "The American role in World War I derived its character less from strategic thinking than from the geopolitical notion that the future well-being of the United States depended upon the balance of power in Europe and the outcome of the war."
The book talks about how once the war got underway in 1914 after a short period of non-involvement the war quickly became "the biggest profit-making enterprise in the history of American exporting" for American enterprises. This included farmers, and bankers. In fact the Allies borrowed $2.5 billion from the Americans, whereas the Germans only borrowed $45 million. I'm sure JP Morgan was a part of those loans in some way. The book American Foreign Relations: A History, Vol. 2: Since 1895 tells much the same story as For the Common Defense. As American Foreign Relations put it on page 76 "[n]eutral or not the United States had become the arsenal of the Allied war effort."
The US had a very strong economic self-interest in the Allies winning the war. Then of course there was the public outrage over the ever-increasing amount of German U-boat attacks, as most publicized in the Lusitania incident. Of course the Germans didn't have much of a choice knowing full well that much of the American shipping they were sinking was probably carrying supplies to the Allies. There was also the Zimmerman telegram incident, but at that point the die had already been cast.
Additionally, prior to World War I the US was in the process of becoming a world power and flexing its economic and military muscle. It had acquired colonies (Puerto Rico, Cuba, American Samoa, Guam, Philippines) and bested the former Spanish Empire in the Spanish-American War, which resulted in the acquisition of all but American Samoa. The country was probably in the right psychological mindset to engage in the Great War.
Economic self-interest was the key factor in the US entering WWI. That self-interest was paired with the actions of Germany (U-boats, and Zimmerman telegram), a growing sense of world-powerness (fueled by colonial growth and economic strength), and a dislike for autocratic systems of government.