When Britain also declared War on Japan, now the US was freed up to give Britain all the supplies she needed, both for Europe and Asia.
Hitler knew that for all intents and purposes, when the US declared war on Japan, it pretty much was a declaration of War on Germany.
Even had Hitler not declared war on the US, it only would have been a matter of time before Germany sunk a US vessel, providing the Casus Belli for the US to declare war on Germany.
The US was already fighting a war in the Atlantic with Germans. We were convoying ships halfway across the Atlantic - ships filled with US built war materials. And we attacked German Uboats. That was an act of war by any rational definition.
Also, the Germans had already sunk US ships - look up the USS Reuben James.
The Germans were willing overlook this if we did. And FDR might have had a problem getting the US to declare war on Germany if Hitler had not declared it first.