Different expectations following the war. "The Cold War' by John Lewis Gaddis goes in depth about this. The Russians expected the Americans to leave Europe after the war and Stalin believed that the conditions of post-war Europe, the starvation and poverty, would help push Europeans towards communism in upcoming elections. The British were in Greece and Turkey trying to hold the states together and keep communist activists from gaining influence, but England was bankrupt and couldn't continue so they asked the US to step in. The US ended up staying in Europe after the war because of this and other matters and initiating the Marshall plan which heavily alleviated the conditions Stalin was depending on to turn Europe to his ideology. one of the arguments made for the Marshall Plan in the US was that it would stem the tide of Communism, actually.

Then there were some hot debates over what to do with Germany following the war. The inability of the US and USSR to come to an agreement on this issue towards the end of the war was indicative of the cracks in the war-time relationship. Fast-forward a few years and you have the Berlin Blockade by Stalin which was combated with the Berlin airlift.

Additionally, though we were fighting the same enemies during the war, it was a relationship built on necessity. The Russians were trying to spy on the Manhattan project and we knew they were trying this. We didn't trust them and they didn't trust us, and this was confirmed for the US as we saw Russia liberate Nazi occupied Eastern European states and then incorporate them into the Soviet Union. This included the Baltic States, which the US never approved of and was a sticking point for the next 50 years.