When World War I began in 1914, the United States did its best to stay as neutral as possible. President Wilson, along with the majority of Americans, initially saw the war as a European matter. In particular, Wilson was worried that the diverse heritage of the United States would lead to divided loyalties should the country get involved directly in the conflict. Furthermore, many progressives in the United States were concerned that involvement in the war would stymie social progress at home.
However, remaining neutral proved difficult, if not impossible, as the conflict progressed. The German blockade of Great Britain threatened international commerce. This only got worse in 1915 when it became German naval policy to attack any ship in British waters through the use of unrestricted submarine warfare. As more ships were attacked at sea, more Americans were getting killed, most notably with the sinking of the Lusitania and the Sussex. Wilson strongly condemned these attacks, and the Germans briefly halted them. However, their decision to resume unrestricted submarine warfare in February 1917 pushed the United States into the war.
There were other factors that led the United States to abandon its position of neutrality. In 1916, Pancho Villa began leading raids on US targets from over the border of Mexico. This highlighted America's military unpreparedness. As the United States began strengthening its military in response, the Germans became increasingly worried about what would happen if America joined the war.
The following year, Germany attempted to deal with this by sending a secret message to Mexico urging the country to attack the United States and reclaim its lost territory there. Mexico (which was dealing with a revolution at the time) was not interested. However, this message, known as the Zimmerman Telegram, was intercepted by British intelligence and turned over to the United States. This made it very clear to Americans that Germany could not be trusted.
Long-standing ties of friendship and cooperation meant that the United States had been loaning vast sums of money to Great Britain before and during the war. By 1917, this had amounted to about two billion dollars. Some Americans felt that the country needed to intervene on Great Britain's behalf in order to safeguard this investment.
Finally, the fall of Tsarist Russia in February 1917 made Wilson's claims that he wanted to fight to make the "world safe for democracy" all that more plausible.
Whether or not American involvement in the war was justified will be a matter of your opinion. We will never know for sure what would have happened if the United States remained neutral. However, the war likely would have dragged on much longer and the victory of the Allies could be cast into doubt. More American lives would have been lost at sea due to submarine attacks, and Germany may have continued other covert antagonistic measures against the United States. Compared to the other combative nations, the United States suffered far fewer casualties. You can ask yourself if you think that any American deaths were worth it in order to bring the war to a speedier end.