I think there is a bit of a misunderstanding by OP in what, exactly, the term "allies" entails in the case of the USSR and the USA during World War II. As /u/jeffdn points out, the USSR and the USA were military allies in the sense that they had a common enemy in Nazi Germany and in that both agreed that the greater threat to either was Nazi Germany (not each other). The only reason that the USA and the USSR got along at all was because each thought the Germans were more dangerous. For ease's sake, I'll refer to the Western Allies for most of my comments, primarily because, unlike the USSR and the USA or Great Britain, the United States and Great Britain were true allies in the traditional sense, coordinating everything from political goals to military commands.

Before the war, the Western Allies and the Soviet Union were not allies by any means. In fact, shortly after the October Revolution, the Western Allies sent troops as part of an international contingent (including Americans) to assist the Whites (conservative, autocratic faction) at Archangel against the Reds (Bolsheviks). This did nothing to endear the Western Allies to the newly formed Soviet state. This intervention lasted about two years (1918-1920).

Stalin, who ruled the USSR from the late 20s through his death in 1953, was a notoriously suspicious person, who never really believed that the US was a true ally of the USSR. If you read FRUS ("Foreign Relations of the United States", basically the official record of US diplomatic communications), he and his subordinates, complain practically from the beginning of American involvement that the Western Allies were trying to bleed the Germans dry with Soviet troops. As early as Spring 1942 (keep in mind the US entered the war in Dec 1941), the Soviets were pushing for the Western Allies to open a second front in France, noting that Soviet losses to the Germans were absolutely huge. For example's sake, Stalingrad, a battle that had just ended about this time, saw more military deaths on the Soviet side in that battle alone than the United States lost in the entire war, Europe and Pacific theaters combined. To make matters worse, Western leaders repeatedly promised the USSR that they would open that front, but repeatedly went back on those promises, saying they were not yet ready. While it's likely true that they weren't yet ready, the opening of the North African campaign in 1942, and then the invasion of Sicily and Italy in 1943 did nothing to persuade the Soviets that the Western Allies were being truthful with their excuses.

This is, above and beyond, all of the machinations of the various regimes at Yalta and then later at Potsdam, which certainly did not help things from the American perspective. Long story short, the USSR promised free and open elections, which they did claim they allowed, but Western leaders considered a farce. The USSR, on the other hand, was basically attempting to establish a buffer zone between Germany and the Soviet Union, given that the Germans had twice invaded Russia/USSR in some 30 years, reeking untold damage and destruction in their wake both times.

So to keep this relatively short, the Western Allies and the USSR were not really allies in the truest sense. They were allies by necessity, allied only to the extent that their common enemy (Nazi Germany) remained. Once that enemy was gone, their alliance fell apart almost as quickly as it began (which I should note started only once the Nazis invaded the USSR in 1941).

Well, the Americans and British were military allies of the Soviet Union due to the fact that they were fighting a common enemy, and it made sense to work together. Even before the war ended, however, there was jockeying for position and spheres of influence in the postwar world. The Soviets wanted Eastern Europe, the Allies didn't want those areas to fall under communist control. Read about the Yalta Conference, it's a fascinating subject.