The US developed the containment policy because of the belief that the Soviet Union was a neurotic and untrustworthy state that only sought to expand in communism, size and economy and US president Harry Truman, who needed to secure his own lot in political favor.
The Soviet's "obsession with security" and desperate need for buffer states and a warm-water port were misunderstood as actions taken to convert more nations into communism. Clearly, the losses that the Russians had to endure were much more than trivial and their struggle to recuperate and protect their people from future invasions more than selfish and expansive. The Soviets lost 20 million people, mostly working men, more than all the other Allies combined, owned a deeply dented economy, and had been invaded three times within the beginning of the 20th century itself. The US government did not ever have to deal with such loss and geographical disadvantages and thus could not possibly empathize with Russia's needs.
Most importantly, the US was concerned about the spread of revolution across Europe, their trade partner and home of their allies; containing communism, a belief system that was everything opposite their capitalism, was the crucial goal at hand.
Furthermore, FDR's replacement, Truman, was kept from foreign affairs and was completely unprepared for the presidency that dawned upon him. He had to secure his position by bending towards the masses that were crying havoc over the Soviet Union's actions and stances. The Long Telegram of April 1945 arrived at apt timing for Truman and administration to use as 'proof' of the communists as a threat to the welfare of the West. Truly, if Roosevelt had to deal with the information in Truman's place, he would be wise enough to check the sources and dismiss the matter altogether. Truman, having...