The United States and the Soviet Union had two different visions for the postwar world.
The United States wanted to create a new world in which all nations had all the right of self determination. They wanted to gain access to raw materials and markets for it's industries. They wanted to rebuild European governments to ensure stability and to create new markets for American goods. They also wanted to reunite Germany, believing that Europe would be more secure if Germany were productive.
The Soviet Union however, wanted to encourage communism in other countries as part of the worldwide struggle between workers and the wealthy. They wanted to rebuild its war ravaged economy using Eastern Europe's industrial equipment and raw materials. They wanted to control eastern europe to balance US influence in Western Europe. They also wanted to keep Germany divided and weak so that it would never again threaten the Soviet Union.
The United states had this vision because not only did they do best as a power when there was peace in the world, but after becoming the economic leader in the world from the war, in order to keep growing, they needed to be able to sell goods to Eastern European countries and gain access to raw materials.
The soviets had their own different vision because of Stalin's love for power and control, and because after losing so many citizens in the war, and their history of being invaded, they felt justified in claiming Eastern Europe to stop future invasions from the west.
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