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...

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.