Stalingrad, the Pacific Island campaigns, to the Ally positions at the start of The Battle of France, it seems that the Allies were more willing to retreat, conduct strategic withdrawals and even surrender (Singapore) earlier in the war. The Russians, too, learned to conduct fighting retreats (just prior to Stalingrad). Especially in the latter half of the war, it seems the Axis were suicidal in holding on to land conquered (in some cases literally). Was this simply desperation? And were such surrenders as Singapore in part due to the knowledge that the Allies would be able to steamroll later, after a long buildup?
In Europe, Hitler's ambitions were the key factor. He believed that whatever nation emerged as dominant in Europe would face a long struggle against the United States. (This was largely due to his racial beliefs; he saw the United States as a strange hybrid of Jewish conspiracy and Anglo-Saxon industry.)
Hitler believed that any truce or negotiated peace which prolonged the war would only play into the manpower, resource and industrial advantages of the Allies and the USSR. If Germany went on the defensive, it would eventually lose. He chose to maintain an offensive posture at all costs in hopes of hanging onto any slim chance of victory. Only by winning at Stalingrad could Germany expand its conquests in the East. Only by winning in the Ardennes could Germany strike at Allied logistics and drive a wedge between the American and British armies. The Germans could exact a heavy price if they stayed on the defensive - but never heavy enough to force the Allies to the negotiating table or bring the Soviets to collapse.
Hitler's conclusions are, in this as in many other ways, debatable to say the least. But that was his reasoning, such as it was.
For the Japanese side of this, you might be interested in a thread that explores the nature of Allied advantages in the Pacific Theater: