Germany lost for the same reason Germany started the war, because Hitler was in charge.  Speer described Hitler as an incompetent, unprofessional, self-taught layman: "Without any sense of the complexities of any great task, he boldly assumed one function after another."

• Invading Russia might be Hitler's biggest mistake but there are other important miscalculations.  Some are possibly more egregious than Operation Barbarossa.

• He declared war on the United States, the single greatest industrial power in the world.  The decision borders on the stupid.  One historian says Hitler made this decision in a moment of absent mindedness.  It is ironic that Churchill's greatest Christmas present came from Hitler.  Churchill said once the US was on board he knew Hitler was finished.

The entire population of the United States, under threat from the Japanese as well, became single minded in their pursuit of victory over both aggressors.  For the first time ever women entered the workforce in huge numbers and proved once and for all they could do a man's work and do it well. 

Isolationism was a thing of the past as men enisted en masse.

While the men fought the women built the war machines.

• Failing to finish the British and French at Dunkirk.  Hitler's decision to halt the advance of armor at Goring's urging who wanted to prove the Luftwatte's mettle.  The decision was mysterious to German high command.  Then Britain surprised the Germans with the most successful sea evacuation in history.  A disaster was averted by Hitler's decision.  215,000 British and 140,000 French, Polish, and Belgians were rescued over a nine day period under heavy aerial assault.  As a result of the inspired rescue British morale soared and the nation was united in a belief they could prevail against the Nazis.  If this wasn't enough the American public was galvanized by the example of British pluck and courage as public opinion swung towards support of Britain.  A solid majority now backed a peacetime draft and rearmament and Roosevelt finally had the public support necessary for the Lend-Lease act.  This was to be the first of many of Hitler's blunders and a far more important one than anyone could have foreseen as two nations, not one, together prepared for war.  Hitler was proving himself to be his own worst enemy and it happened here, at Dunkirk.

Brits rallied using any boat available to bring home their boys.

Night became the preferred time to cross the channel.

• Hitler failed to defeat the RAF when he had it on the ropes.  Churchill bombed Berlin and an angry Hitler retaliated by bombing London.  Had he continued his strategy of attacking the RAF he may have succeeded because he was close to success.  The decision to switch his air invasion from the RAF to London might have lost him his chance to defeat Britain and possibly win the war.  This one error in judgment, and a pattern of stupid decisions is emerging, might be his most critical mistake.  One reader says Hitler could never defeat the RAF but I haven't found sources to support that.  I did find one saying the RAF had a bigger problem with a lack of trained pilots than a lack of airplanes. Strategic bombing during World War II
• He refused to listen to his Generals, as talented a staff as any dictator could wish for, and as a result made horrendous mistakes.  This character flaw of trusting his own uninformed opinions over the counsel of his experts may have cost him the war.
• He delayed his invasion of Russia by over five weeks after the planned launch date.  This came back to haunt him heavily later.  One reader says a late spring is the cause of the late invasion but I haven't found supporting sources.
• Hitler ignored staff advice that the invasion of Russia would be an economic disaster for Germany. Operation Barbarossa
• Hitler and his staff grossly underestimated the Soviet Union's industrial might, its ability to mobilize troops, and its weaponry.  The Germans were stunned when the Soviets rolled out a capable tank that could outgun their tanks.  The PPD­40 submachine gun followed by the inexpensive PPSh with its high rate of fire and large magazine turned the Soviet soldier into an aggressive and deadly fighting machine.  Because the weapon was only suitable for close quarter fighting it brought the Soviet soldier out from behind their barriers to engage the enemy while snipers took out German soldiers from a distance.

At a thousand rounds per minute the PPSh41 was deadly leading Germans to pick it up for their own use.  In fact it was so popular the German high command developed a program to modify abandoned weapons to use German ammunition and reissued it to their troops as the MP41[r]. 

The stick magazine differentiated the German MP41[r] from the Russian version and helped prevent panicked Germans from firing on their own troops