Why didn't the Yalta conference lead to lasting unity among the big three leaders in World War 2?

Yalta Conference didn't unite the Big Three because Stalin's beliefs in communism and Truman and Churchill's beliefs in democracy didn't go hand-in-hand. Democracy and communism are two things you don't mix together. Tensions between the two superpowers, the United States and the Soviet Union, were imminent after World War II which lead to the Cold War.