Why did the USA develop the policy of containment?

Before the end of the World War Two, the relationship between the US, Britain and USSR went on very well. However, the attitudes of USSR and US had a big U-turn from the Potsdam conference since US saw the communists as a big threat to them. Eventually by early 1946, The Long Telegram laid the basis for the policy of containment against communism.
One specific reason why there was such a big change from the Potsdam conference is that the leaders of Britain and the US were different. Harry Truman who had been working as the vice president before the death of Franklin Roosevelt was now the…