As of December 5, 1942, being drafted was the only way to join the US armed forces (as an enlisted man). Voluntary enlistment was not allowed.
I believe World War II was more popular in the American psyche after the fact, while not so much prior to the war's outbreak.
The American public was pretty isolationist at the time, and when war broke out in Europe, the general consensus was that the U.S. should stay out of it. It was just another "European" war and after the disastrous consequences of the First World War, no one was too anxious to leap into that quagmire again.
All throughout the 1930s, a fierce debate raged through the U.S. regarding pacifism and neutrality in international affairs. Pearl Harbor and the fall of France both significantly altered the general viewpoint in the country but not so much as to negate the need for a draft. The United States had one of the smallest standing armies in the West at the time and there was no way that the recent surge in enthusiastic youth could make up for the gap between the American armed forces and those of, say, Germany.
Source: "Roosevelt's Secret War: FDR and World War II Espionage"