Actually,
Hitler is more logical and rational than OKH and generals in the field. He realized the war was an economic one, and that denying the soviets oil and the grain of the Ukraine would have devastating effects. In late 1942, according to some sources, there were few dogs and cats wandering around in Moscow because people were eating them. The Soviets needed food in Lend Lease more than anything else, more so than a second front by the Allies.
You can see it here on the TIK video’s:
How close was the Soviet Union to Collapse in 1942-1943? - YouTube
For more detailed info about Stalingrad see the links and my post on the TG forum here:
Battlestorm Video Series on Stalingrad - The TimeGhost Army