Maybe the most fundamental cause of the Cold War was the stark ideological difference between the United States and the Soviet Union. The United States was a republic, with a representative government, and more importantly a capitalist economic system, while the Soviet Union was, by the time of World War II, a dictatorship, run by a single party, striving toward communism as an economic system.
Secondly, Stalin saw the end of World War II as a chance to expand Soviet influence into eastern Europe. Whether one views this as a reasonable aim, an attempt to establish a buffer zone to avoid another catastrophic invasion of the Russian homeland or an imperialist effort to expand, or both, it struck US President Harry Truman as the former, and exacerbated tensions between the two powers.
Third, and related to the second cause, serious disagreements emerged over what postwar Europe would look like. The United States, eager to supply European markets with American manufactures, hoped to expand their influence into Germany, in particular, a development that struck Stalin as US imperialism. The Marshall Plan, with its billions of dollars in aid, and the policy of underwriting the German Deutsche Mark, signalled a permanent chill in US-Soviet relations, and by early 1947, Stalin was not only launching new Five Year Plans, but was declaring capitalism and communism to be irreconcilable.