Maybe it's good ole American anticommunism but I've always just assumed that Moscow's objection to the Soviet Union and those new satellite states in their sphere of influence denying the package that came with the Marshall Plan just had to do with Stalin's hubris and stubbornness, or perhaps some sort of nationalist-slash-Marxist-Leninist stand against the U.S. imperialists who were only offering the package to subvert the influence of popular Western European Communist Parties in places like Italy and France.
What was the real reason why? What was Truman's administration offering that would have been un-negotiable to Molotov and Stalin?
The Russians were scared of Germany increasing its power because the Marshall Plan was aimed at building up the German economy and getting it back on its feat. The spheres of influence solution that was decided upon in the Potsdam Conference earlier was falling apart because the Russians were aggravating the Americans by continuing to occupy Iran/Turkey even after the war because they wanted to build a canal - it showed America that an amicable divorce might not be possible.
This led to the western strategy with a formally western/eastern German state, Molotov Plan (which was the Soviets version of the Marshall Plan but for communist states like Italy), the Berlin Wall etc.
It was mostly a security dilemma and not really ideological differences. Russia is afraid of Germany gaining power again, America wants to leave Europe so they build Germany (under Eisenhower), Russia reacts by building up its own power and so on.
*Also, I believe Stalin was open to discussing the plan at first, but America wouldn't give in to some of his demands about how the money would be dealt.