The Soviet Union was attacked by Germany and the Axis. Stalin's ONLY option in 1941 was to commit millions of troops to a war of national survival. The Red Army's only hope of defeating the Axis was to commit troops in overwhelming numbers. Wherever and whenever the Wehrmacht fought any other armies in roughly equal numbers they won. So overwhelm them by sheer weight of numbers, or lose.Well you go and cheer on a dictator who has opposition politicians murdered outside the Kremlin, attempts to murder others and cracks down on dissent and is making life incredibly difficult for ordinary citizens (rather like that bastion of freedom which was the USSR).
The same USSR which signed a non-aggression pact with the nazis in order to carve up Poland (and execute a large part of the military). The same Stalin who accepted much aid from the US, ignored intelligence and threw troops to die like confetti. Their sacrifice should never be forgotten but many should never have perished.
Having defeated the Axis Stalin's obvious motivation was to invade western Europe as well. FDR, Truman and Churchill realised this. The outcome of the war in Europe was decided before D Day, and having left the Soviet Union to fight alone, it's pretty obvious that D Day had to happen not to defeat the Axis but to make sure that the Iron Curtain didn't come down further west than the Eastern border of Germany.
As it was the western Allies struggled to make much progress and consequently two thirds of Germany (1939 borders) fell behind the Iron Curtain. Poland was abandoned for a second time, and Czechoslovakia, Hungary, Romania and the Balkans were abandoned to their fate.