The suspicion was due to the nation's having differing ideologies as well as being the two rival superpowers with different visions of the world. The Americans wanted a capitalist democratic Europe to be rebuilt while the Soviets wanted communism to be implemented. The two powers often butted heads during peace conferences over many issues and only worked together because they were forced to when it comes down to it. If you want to look at this in more depth watch The Last Days of World War 2 it is a good series of documentaries that deal with this topic and others.