Some above have been close to my view on Hitler and the Japanese.

IMO the best guess is that he knew the Japanese would never attack the USSR if he didn’t declare war on the US.

Second guess: He figured the US would get in the war against him by one means or another anyway, so why wait. If he thought that he was right, it was only a matter of time.

Third best guess: He wanted to show his few allies that he would keep faith with them. And perhaps it would sway Franco too. It probably did sway Franco, to conclude that Hitler was doomed.

Personal guess: Hitler craved constant excitement and was convinced boldness brought luck. What is more exciting than declaring war on a major power. It had been a few days……