Greed.
It will get you every time.
Hitler believed that we would fall quickly to the Japanese and wanted to make sure he could grab the east coast of the US and maybe a good chunk of South America.
So he declared war.
Probably the stupidest move he made in a series of stupid moves.
One way or the other, we would have gone to war with Germany soon after Pearl Harbor. There were already plenty of insinuations that Pearl Harbor was done with the cooperation of the Nazis.