One is the good democratic fight versus the bad facists. US were hampered by the isolationist views held by quite a number of senators at the time, hence there was no direct involvement until after Pearl Harbour attack, which resulted in an almost uniform senatorial agreement on declaration of war.
Another is that US could have kept maintaining the profit stream from arms-dealing to both sides (Germany still imported American arms prior to 1941 or so, if I'm not mistaken) but got unwillingly dragged into the war.
Yet another more cynical view is that after the huge Axis loss at USSR, US just saw the opportunity to join in and mope up what's left of the Axis since winners get to dictate the terms. The Pearl Harbour event was a surprising but timely excuse.