The sinking of ships reason was for public consumption, an appeal to fear and pride which is the tool governments use to garner support for their entry into a war on which they decided long ago and invariably for different reasons.America got involved in ww1 because Germany was sinking ships with americans on them, that was the main justification. There were many other reasons but the primary reason average americans supported the war was because Germany was viewed as a threat to America. Everything else was secondary.
The United States government entered the war for four reasons. Firstly, the United States was well on the way to being a world power and by entering the war they were well aware they would take their seat among the world powers, thus reinforcing world power status. Secondly, the United States could not afford Britain to lose that war given the money we had borrowed from them. Thirdly, the United States government wanted to change world politics according to their view of how it should work: chiefly a more democratic approach to international politics which included national sovereignty within and among nations. Fourthly, the United States wanted Europe powerful like-minded friends in Europe and this would aid their inevitable move to being the world's first power, and that meant Britain winning that war.
All of these reasons amounted to one thing: the United States becoming the world's first power.
As for Germany being viewed as a threat to the United States, not a direct military or economic threat, they were only a threat in the sense they had the power to cause problems for like-minded European nations who in the future would support them. They were looking 20 years ahead, and the United States obviously needs like minded friends in Europe so that European policy is more or less aligned with US policy.