Answer:
The primary reason the United States and Great Britain agreed to Stalin's demands for "friendly" governments in the Eastern European states liberated by the Red Army was the presence of the Red Army in those states.
Step-by-step explanation:
During WWII, the Allies doubted whether to trust Stalin or not since he had signed the Nazi-Soviet Pact with Hitler in 1939. On the other hand, Stalin was upset with both the U.S. and Great Britain for their delay in conforming a second front to battle the German army in Western Europe (something which helped Hitler to center his forces to attack the Soviets).
And since Stalin feared that the U.S. and Great Britain could abandon the war (weakening his country), he planned a geographical defense of friendly Eastern European countries so as to protect the nation from western attacks. This defense was originated from the lands the Red Army invaded when reaching Berlin.