The Soviet Union had fought alongside America and Britain, to defeat Nazi Germany, Italy and Japan in the Second World War. At the end of the war, devastation was wide-spread. The Soviet Union was terribly affected by war, ruined by war. Total Soviet casualties doubled those of any other nation. Millions were homeless, without husbands, without an economy to get them back on their feet. Now that the war had ceased, the USSR and United States had no reason to co-operate as they had previously.
Instead of being allies, Stalin was now "deeply distrustful of the intentions of the West... " Stalin was determined to protect his country from an attack by Western powers. The Soviet Union was weak and vulnerable, and its leader, Joseph Stalin, was determined to build a both economically and militarily strong, secure nation. He would do anything in order to accomplish this goal, even if it led to protest, and attack, which can be interpreted as aggressive acts.
Joseph Stalin's foreign policy partly involved strengthening Soviet influence and preventing an invasion from any western country. In order to protect the USSR, Stalin wanted to establish pro-Soviet governments surrounding the Soviet Union. These pro-Soviet governments would act as buffer zones against another attack; Russia had previously been invaded three times by western countries - twice by Germany, and in the Russian Civil War, causing substantial losses of lives and resources. Therefore, Stalin desired to safeguard Soviet security.
These 'buffer zones' would "act as a barrier against further invasion of the Soviet Union from the West, " providing security for the USSR. He was determined to never allow such a devastating war and its terrible effects to destroy his country like it had after World War Two. At this point, it is understandable why Stalin would have wanted to defend his country from what he saw as threatening western powers. The United States did not understand Stalin's obsession for security, and thought he was trying to spread communism, especially when he wanted to create surrounding pro-Soviet states.
I agree that Stalin's foreign policy in Eastern Europe was defensive, but only up to a point. In his heart, it seems as if Stalin had not abandoned the idea of expanding world communist revolution. After the post-war years, when the Soviet Union had had time to recover from the effects of war, Stalin's policy changed from defensive to aggressive. This policy even led to the point in which Stalin wanted no intervention at all from the West in Eastern Europe. He aggressively and desperately desired total control, to dominate the whole of Eastern Europe and establish communist ideology.