The main cause of the Cold War was the fact that the communists and the West did not trust one another. Each felt the other was going to try to destroy it. Each felt the other was going to try to impose its system on the whole world.
The West felt the communists were going to try to foment revolutions in every country or to simply conquer countries and force them into communism. The USSR had already done that in Eastern Europe and communist doctrine calls for the whole world to become communist.
The USSR felt that the West was going to try to destroy it. Western governments had been anti-Soviet since the creation of the USSR. The United States, for example, had sent troops to the USSR after WWI to help the anti-communists in the Russian Civil War.
The Cold War happened largely because of this mutual distrust. Follow the link below for a much more comprehensive discussion of this topic.