The major factor is that they had different ideologies.
While the United States(US) believe in capitalism, where people should have freedom on their daily basis,such as trading,politics, and speech. It encouraged a free Market. While Soviet Union(USSR) took up communism, which controlled the general market and people do not have the freedom of speech and politics.
Both had the fear of the spread of one another's ideology especially US,was worried that communism would spread to the western Europe.Thus causing the tension and much suspicion between the two nations,leading to Cold War.
Hope it helps!
