When World War II ended, it was somewhat of a mystery to some why tensions between the Soviet Union and the USA suddenly rose. Communist Russia had proven not to be a threat to the West during Lenin's reign, but most credited Nazi Germany with keeping it at bay rather than lack of ambition. The USA and Soviet Union had been allies during the war.
However, the differences and incompatibilities between the Economic structures of Communism and Capitalism, and the necessary Governmental structure to maintain either one quickly tore apart the cooperative spirit that had held the two together during the War. With the buffer zone of Nazi Germany gone, and many of the countries of Eastern Europe copying Soviet Communism, the USA and Britain began to fear that they had traded one tyrant for another.
As the 50s launched the space and arms races, the nature of the Cold War began to change into a contest between Capitalism and Communism, rather than a political conflict between the dictatorial nature of Communism and Democracy. The 60s and the rise of socialistic ideals of egalitarian economy, and hippie culture moved it to a purely political conflict, between dictatorship and democracy. Vietnam and the Arms race moved the 70s towards a cynical view of the tension as being unnecessary, and belief that fears of spreading communism were exaggerated. The 1979 invasion of Afghanistan, brought back a fear of Communist expansion, and culminated with the Peak of the Arms race. Ultimately the Communist government couldn't keep up with Capitalism and the Cold War ended. Capitalism crept up throughout the Soviet bloc, and tore down the dictatorships, resulting in the current European situation we have today.
Another reason, besides the evolving perception of the Cold War, has been declassification. Many details of immediate Post WWII arrangements, and US Military innovations were classified at the time, as were many of the flashpoints of the Cold war: Korea, the Cuban Missile Crisis, Vietnam, etc. As those details became de-classified, new insights into the causes of the Cold War were discovered, and a new understanding of the conflict emerged.