Stalin really thought Hitler would keep his word, and seemed to be poleaxed and unable to lead in the early phases of Germany's Barbarossa -- Molotov was sent east for meetings with the Japanese, and they were not fired up about another meeting / beating with the Red Army. Molotov wired back to Stalin that he believed the Czarist-era treaty would hold. That led to the transfer of 70+ divisions out of the east (which is where the first stroke of war had been expected), with the trains loaded to extreme capacity to move all the armor and big guns and everything else to the USSR's western front, and returned east for more, completely empty. That led to the first effective counterattacks.
For his part, Hitler was just a dumbass wanting a do-over of WWI, with the change that one front (France etc) would be finished up first, allowing the use of most forces against the Soviets. Obviously that was just brain-dead. The only move was to finish off the British in North Africa and the Mediterranean, take control of the Suez Canal and access to the Middle East oil. By the time 1943 rolled around, and Germany was finally fully mobilized, *perhaps* it would have been feasible to invade the USSR. But then again, why? What could have been gained, conquering a poverty-stricken nation of agrarian serfs?
Hitler needed the oil of the Caucasus and resources of the Ukraine. The British blockade was really hurting Germany and Hitler decided he needed those resources to continue to dominate Europe and carry on with the war.
Plus, there was that lebensraum nonsense. Hitler believed the Malthusian crap and concluded he needed to expand Germany east into all of Poland and Western Russia (and eventually exterminate most of those poor Poles and Russian serfs).
Had Hitler waited until 1943 to invade Russia, Stalin probably could have finished his rebuild of the Red Army and established good defensive lines in "his" part of Poland, which he had not in 1941.