The fundamental cause of the tension between the US and the Soviet Union and, therefore, of the Cold War, was the fact that the US was a capitalist and democratic society while the Soviet Union was a communist society. These ideologies were incompatible with one another and this caused tension between them.
The tension between the two countries started as soon as the Soviet Union came into existence. The US did not want the Soviet Union to exist. It actually sent troops to help the communists’ opponents in the Russian Civil War. It refused to recognize the Soviet Union diplomatically until the 1930s. The US was suspicious of the Soviet Union because of the fact that it was communist. Communism was an ideology that was abhorrent to American ideas of freedom and liberty. In addition, the ultimate goal of communism was to dominate the entire world. This made the US suspicious of the USSR.
For its part, the USSR was suspicious of the US and of the West. This was partly because of the fact that communism saw capitalists as oppressors. The suspicion was exacerbated by WWII. This led the Soviets to want a buffer zone in Eastern Europe between themselves and the West. This desire helped to convince the US that the communists were trying to spread as a first step to trying to dominate the world.
For these reasons, the US and the Soviets drifted into the Cold War almost as soon as WWII had ended.