1. Hitle delcared war on the US.
That's one thing I've never seen explained btw.
It's more like, while the US and UK involvement was needed, the SOviet Union is who won the war.
As to the general question, I disagree there was a "clear" good side. There was a "clear" bad side along with a murky baddish side, which is different from most wars which just have two murky baddish sides, and there's no denying I would have "taken sides" wherever I was, which is more than can be said of just about any war fought in my lifetime. Which is near enough the original question I suppose.
Hitler wasn't really all that bright in the strategic sense and Germany has never been very skilled at diplomacy (at least not since Bismarck).
I don't think there is very much in the way of a rational explanation, to be honest. I think he wanted to stand with the Japanese or something and in his worldview the Americans probably couldn't be much of a threat anyway.