And for my 'personal' reason why Germany lost on the East, it's because of his obsession to Stalin, basically, Hitler's quite a douche taking a not-so-important city with Stalin's name on it and cost him 1 army. Why not call him Obsessed?
The Russian winnter was very... nasty very cold, an d the military police of the Nazy Germany were realy pack of idiots. For more detalys investgate Sven Hassel's books. There it's only about the Members of the Heer(german ground army) and theyr's mizerabil (or not) lifes