Differing Ideologies:
Communism vs. Capitalism - One of the main reasons that the United States and the Soviet Union had clashed after World War II was over ideologies. The Soviet Union wanted buffer nations to protect itself against any possible attack, and to create these buffer nations, the Soviet Union had to convert them to communism. The United States disagreed with this idea, and believed that if these nations were created, then communism could just continue to spread until it reached the United States.
Weaponry:
Weapons - The United States developed a nuclear bomb during World War II, and the Soviets were wary of this. Truman let it slip that the United States still had weapons of nuclear destruction which led to the Soviets fear of being bombed. Stalin was upset that the United States wouldn't tell about how far it had gotten in nuclear development, so Stalin tried to develop weapons too to compete with the United States.
Communist Expansion & Broken Promises:
Communism Expanding - The United States opposed communism starting way before the Cold War. This notion only became worse when the Soviet Union was spreading communism slowly though Europe, Asia, etc. To add insult to injury, when the Soviet Union promised to allow countries to choose whether or not they wanted to be communist, they broke this promise and rigged elections in communism's favor.