Having lost the Battle of Britain and being required to give up Operation Seelowe, Hitler had only one possible place to get the oil that he needed.
Operation Barbarossa wasn't (as is often supposed) an act of madness on Hitler's part. It was literally the only chance he had to win the war.
It's a fascinating piece of history, which remains highly relevant today.
I don’t know for sure if I agree that it was the only reason; I have skimmed other sources (but not read or considered in detail enough to know if they are right) that Stalin was preparing to backstab Hitler by invading to the West, when Barbarossa was launched.