Though Japan, not Germany attacked the United States on December 7, 1941, there was no particularly good reason for the United States to retaliate by attacking Japan before her German ally. Similarly, if set upon in a dark alley by two thugs, there is no reason to attack first the one who originally assaulted you. Roosevelt had three excellent reasons for the "Europe First" strategy.
First, a need to support the USSR by opening a second front. As soon as the Germans opened operation Barbarossa by attacking the Soviet Union, the damage wreaked against the Soviets was quite terrible. Though the British put up a noble defense in Western Europe, they occupied few German troops, freeing Hitler to attack Stalin with 3,300,000 initial troops. The longer the Russians had to bear the brunt of this mechanized attack, the greater the chances that they would attempt a negotiated settlement with the Germans, a solution which would spell disaster for Britain, and increase the German threat in Africa and South America. Far better for American troops to reopen the second front, showing support for Stalin and drawing German troops away from Russia.
Second, Roosevelt and his aides wanted to assure themselves of the continued independence of Britain. In part this stemmed from purely emotional attachment to the British and their leaders. In part, Britain served as an excellent launching point for what was the probably inevitable move against Germany. Therefore, landing troops and equipment in England was a wise investment. Leaving the troops there while moving against Japan made little sense; a full-fledged attack from Britain against German troops on the continent would serve the purpose of protecting the island and engaging the enemy without inefficiently sequestering troops where they would not be used. In the end, the U.S. proceeded against both the Japanese and European fronts with about equal resources, but the importance of protecting the continent theoretically allowed more intelligent allocation of limited resources.
Third, Roosevelt believed, probably correctly, that the Germans would continue the war whether or not Japan was able to continue fighting and furthermore, probably erroneously, that the Japanese were likely to capitulate as soon as the Germans surrendered. Therefore, a victory in Europe might well kill two birds with one stone, saving much money and many lives.
from my homework for Historical Studies B-54 on October 16, 1991