From: Bremen, Germany
The second thing was the Germany and Japan had the tripartite pact which was a defensive pact primarily directed against the USA. Despite Japan bombing Pearl Harbor, it was the US that declared war the next day, activating the pact. This probably was just a justification for reason #1, but in any case things aren't as simple as "he was stupid he did a blunder".
Nope. It was a another gamble from Hitler. He hoped that if he declared war on the USA, Japan would do him the same favour in regard of declaring war on the USSR.
Hitler was a high risk gambler. Declaring war against the USA gave him a short term advantage ("happy days" for the german Subs) and the chance, that Japan might draw russian resources toward the far east. Maybe he even knew in late 1941 that the jig was up, so he went all in.
"You will be dead, so long as you refuse to die" (George MacDonald)