The fundamental reason for the US entry into WWI was the fact that the US had strong economic connections to Britain. These were strong before the war and became stronger as the war went on and England needed more goods. These economic ties helped to make Americans see the British as important allies.
There were other issues that helped bring the US into the war. The unlimited submarine warfare waged by the Germans helped to bring the US in. This was connected, of course, to the US desire to continue to trade with England.
The US was also angered by the Zimmermann Telegram. This was an offer by Germany to give some US territory back to Mexico if Mexico were to join the war on the German side. This made Americans more likely to take the Allied side. IN addition, there were reports of German atrocities that upset many Americans.
However, it was largely the economic ties to England, along with the idea that freedom of the seas was an important right of neutrals, that led to US involvement in this war.