What specifically caused the U.S. to join the Allied side in WWI?
This question has been asked and answered before -- see link below. However, I think that there is one very important factor that is not addressed in any of the answers in the "q and a" link below.
The factor that is not addressed there is the fact that the United States had so much more trade with the Allied countries than it did with the Central Powers. This was true before the war and it became even more true as the war went on and the British blockade of Germany took effect.
Because of this trade, there was a great financial incentive for the US to side with the Allies. If the Allies lost, the money owed to US companies by the Allies would be lost as well. This would give Americans a powerful incentive to side with the Allies.
We can see that Americans of the time thought that this had helped to cause US involvement in the war. This is shown by the fact that the Neutrality Acts passed in the 1930s to prevent US involvement in another war prohibited trade with belligerent countries. Congress did not want the US to have such a financial incentive to go to war again.