It was Realpolitik, as usual. The Germans and Russians had much to gain by cooperating in '39 and '40, and much to lose by not doing so. So they temporarily put aside their mutual loathing and cooperated with each other. That's typical of the strategic maneuvering that competing nations engage in. Great powers do not have friends, they have interests.

Hitler was an idiot to attack Russia in '41. He should have left them alone. They would certainly have left him alone.