First, The United States had a tradition, begun by George Washington, of staying out of Europe's wars and troubles. This stance, called isolationism, had helped the young, weaker country focus on its own problems and to grow strong internally. Even while the United States was poised to enter the international arena in the twentieth century as the world's superpower, many people still clung to the "mind our own business" stance. In fact, Woodrow Wilson won re-election in 1916 partly on the basis of having kept the US out of World War I.
Another obstacle to entering the war was the large number of German-American citizens in the country. Though it seems inevitable now that we would side with the English, at the time that was not a given. It was not until the sinking of the Lusitania by the Germans, which killed more than one hundred Americans, and the revelation that Germany had approached Mexico about an a alliance against the US, that the American people could be persuaded to go to war against Germany.
As for actual recruitment and mobilization, as Howard Zinn outlines in his book A People's History of the United States, the government had a hard time getting enough men to enlist in the army and had to spend more money on propaganda than it had intended. There were also three rounds of the draft, and registration for the draft was required even for men who were not citizens. This suggests that, despite all the efforts of the government, for many people this war did not feel like "our" war.
Finally, the government was forced to make concessions to the labor movement to prevent strikes that would have delayed the production of weapons and equipment to fight the war.