The leaders of the US, USSR, and Great Britain said they wanted to cooperate, so why were negotiations at the Yalta and Potsdam conferences so difficult?

Actually, the three great superpowers at Yalta did want to cooperate, because they wanted to ensure peace for the postwar world.  However, they all had different agendas to promote.  Stalin acted on his own and was not responsible to the people of the Soviet Union.  Stalin wanted to ensure a buffer zone between the West and his country, because the Soviet Union lost heavily in terms of men and material in this war.  Churchill openly distrusted Stalin and there was still some animosity between Britain and the Soviet Union over the Nazi-Soviet Non-Aggression Pact in 1939.  Stalin wanted to see the prewar government of Poland restored, but Stalin wanted his regime to remain in place.  When Churchill complained that Stalin was expanding his borders, Stalin pointed an accusing finger at British colonies and asked why the British could have a sphere of influence and the Russians could not.  Roosevelt, on the other hand, needed an assurance that the Soviet Union would stay in the war to fight the Japanese, who were already retreating in January 1945 but were still quite strong.  Roosevelt was willing to agree to almost anything to get this assurance, even if it meant that Eastern Europe would be occupied by the Soviet army, which was already a reality on the ground at the time of the conference anyway.  Roosevelt thought that he could negotiate a deal with Stalin to get the Soviet Union to acknowledge the prewar territorial boundaries, but the American president died in April before the war's end.