Why was the Soviet Union upset with the Western Allies?

It was because of long term and short term factors.

Although the Soviet Union and the West were allies against Nazism, this was an uneasy alliance.

For a start, even before World War 2, the West had attacked the Soviet Union or Russia as it was known twice in the 20th century. The Germans attacked in 1914 and the West supported the Czarist White Armies against the Bolsheviks during the civil war after 1917.

The west, both capitalist and fascist, continued their hostile attitude throughout the 20's and 30's. This made the Molotov-Ribbentrop Pact all the more surprising. However it was an agreement based on expediency for both sides as Stalin knew at some point Hitler intended to attack.

The Pact meant that when war broke out in 1939, the Soviet Union and Nazi Germany carved up Poland between them. When Germany defeated France in 1940 Stalin instructed the French communist party not to oppose the Germans. The Soviets also supplied important war materials to Germany right up to the day of the German attack on June 22nd 1941.

This backdrop shows how complex the situation was. In 1945 although the Soviet Union was on the same side as the allies, as soon as Germany was defeated (in fact before the war ended) these historical divisions emerged.

From the Soviet point of view the policies of the Western allies reflected their historical suspicions. During the war Stalin constantly called for a second front to help the Soviets who had absorbed over 80% of the German war effort. The fact that it didn't happen until June 1944 re-enforced his view that the West wanted the Soviets to absorb the German war effort.

It is also the case that strategies such as the Marshall Plan and the re-integration of Germany into Europe alarmed the Soviets. These factors accelerated the division of Germany and Berlin. When West Germany became a member of NATO in 1955 the Warsaw Pact was formed.

Remember the question asks for an explanation of the Soviet perspective. From a Western perspective all these arguments could be reversed.