[@Inertia] Because promises made to Stalin during the Second World War were not delivered on and it made Stalin paranoid and suspicious. The Americans in turn didn't enjoy what the Soviets did after the end of the war and their general policy of disliking Communism didn't help much either.

(Stalin asked and was promised a second front to open in 1942, he waited as more and more Russians died. Then he asked again in 1943, again the allies promised him a second front would be opened soon. More Russians died yet again, empty promises.) Some people even speculate that the allies did this on purpose to weaken both powers enough so that things would go more in their favour, yet Soviet Russia had some real ass kicking abilities.

Not counting Italy because it didn't really put much pressure on the Germans untill later. (D-Day in France.) 

Edit: Stalin was already paranoid and suspicious, the allies just poured on top of that. But you can't blame him, look up Operation Unthinkable for an interesting read.