To add to the above response, which covers it pretty well, in terms of trade, during the first nearly three years of war in which the United States was officially neutral, American trade with the allied nations increased steadily while trade with the Central Powers dropped off to almost nothing. So while we were "neutral", Kaiser Wilhelm of Germany knew otherwise.
He also suspected that within many of these cargo ships, and even some of the passenger liners, were stored massive amounts of military supplies bound for her enemies. It was the equivalent of someone saying they are not taking sides in a fight even as I see them hand my enemy a gun. To him we were already in the war, and this led to unrestricted submarine warfare as official policy. It is unclear whether the Lusitania was transporting war materiel or not.
The link below is a good summary of causes for US entry as well.
It's not clear to me what you're asking, but I've edited your question to what I think your'e trying to say. I'll give a brief explanation of the main causes you cite...
1. Submarine warfare. Germany practiced "unlimited submarine warfare" meaning they'd sink any ship bound for England without stopping it first to make sure passengers got off safely, etc. This (especially the sinking of the Lusitania) made Americans angry.
2. US trade with Britain and France. The US traded much more with those two countries (especially Britain) than with any of the Central Powers. This was true before the war and it became even more pronounced once the British blockaded Germany. Some people say this helped get the US into the war to protect its economic interests.
3. I'm not going to talk propaganda because I don't think it's that importan. But I do think you need to think about the role of the Zimmermann Telegram (trying to get Mexico to declare war on the US -- offerring to return the Southwest to them after Germany wins the war).
If this isn't what you need, please clarify your question.
The sinking of the Lusitania was one of the major factors as the British ship carried American passengers, which resulted in the USA participating in the war.
The last answer by 'revolution' is incorrect. the 'Lusitania' was sunk on March 7th 1915, Unrestricted Submarine Warfare began on February 1st 1917 and the US declared war on April 6th 1917. The sinking of the 'Lusitania' made Americans angry but since Germany paid the American families for their losses and said they would not do this again all was forgiven. However, German did do this again with the sinking of the British ship the 'Arabic' (8/15) and the French ferry the 'Sussex' (4/16), so to end this conflict Germany agrees not to sink British passenger ships, only cargo ones.
For some reason though, Germany decides that even though it is winning the war (Russia has dropped out and the French are pushed almost to Paris) that it will declare Unrestricted Submarine Warfare which meant that they would blow up all ships in the warzone whether they were allied with Britain or not.
Finally, the Zimmerman Telegraph/Note is what really pushed the US to war. This stated that "we (Germany) intend to begin unrestricted submarine warfare on the first of February. We shall endeavor in spite of this to keep the United States neutral. In the event of this not succeding, we make Mexico a proposal of alliance on the following basis; make war together, make peace together, generous financial support, and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico and Arizona."
Although Mexico declined Germany's offer, this promise of US territory to Mexico was a direct threat even though the US was not supposed to know about this (Britain intercepted this telegraph and passed it to the US) because it said that if the US joined the war then Germany would take its land and give it away.
So after Unrestricted Submarine Warfare and the Zimmerman Telegraph the US had no reason other than the usual (death, cost, time, etc) not to go to war with Germany. The war was also a way to strengthen its ties with Great Britain and assert itself as a Great Power in the Great Power Rivalry.
I hope this helps!
The main reason why USA joined the war, due to the fact that Germany had sunk one of the British ship, which is called Lusitania, which had American passengers. This angered many Americans, who decided to ask the government to start war with Germany, thus beginning their involvement in the global war. This is Germany practice of "unlimited submarine warfare", which involves the sinking of ships right under the radar of vision bound for England. This proves to become a pivot of the USA's entranced into a bloody war.