One of FDR's boldest moves in the early stages of his presidency was to open relations with the Soviet Union in 1933. Roosevelt understood quite pointedly that Germany had aggressive designs in terms of land acquisition and the consolidation of power. He also understood that Japan had the same ambitions. Seeing that Americans were steeped in the midst of the Great Depression as well as the isolationist thought which dominated the time period as a result of the shadow of the First World War, Roosevelt understood that he lacked the political capital and resources to be able to commit to U.S. Intervention in the region. Americans were simply not having it. In the final analysis, opening relations with the Soviet Union helped to ensure that Roosevelt could have an ally if the expansionist dreams of Germany became too large and threatened nations' sovereignty.
The only time the United States appeared to have civil relations with the Soviet Union was during the 1930's and 1940's. Prior to that time, the US had actually backed the "White" Russians during the Russian Revolution which began in 1917 and continued into the 1920's. Unfortunately, that was the losing side, and the new communist government didn't forget it. Great Britain, on the other hand, had watched with growing interest the Soviet Experiment, and believed that communism was indeed the way of the future; much of Soviet business, industry, and technology was seeded by them. In fact, counter to the remainder of the industrialized world, the Soviet economy expanded during the Great Depression. By 1933, during the nadir of the Depression, Germany elected a new leader; the US saw more danger in Hitler than Stalin, and for the remainder of the Depression and World War II, the Soviet Union under "Uncle Joe" was our best ally. After the World War, the Cold War began, and the US and USSR remained at odds until 1989 until the Soviet Experiment was no more. Politics makes strange bedfellows.
I would say there are a number of reasons that the US (and Pres. Franklin Roosevelt, spefically) decided to open relations with the Soviet Union.
- The United States was the only major country that still did not have relations with the USSR at that time.
- The US was worried about Japan's expansion in Asia. It thought that the USSR could act as a check on Japanese ambitions.
- The US hoped to be able to trade more with the USSR -- remember that this was during the Depression and we really needed ways to get more money.
- FDR was trying to pursue a generally "nicer" foreign policy -- as with his Good Neighbor policy in the Americas.