Edited to remove the summaries. I want to encourage people to read the linked comments, not my truncated snippets.

Mentioning previous answers is not intended to shut down more questions and discussion. Most posts on this subject have been archived, so you can't ask followups there. So please feel free to post more discussion or questions here.

In a quick search of this subreddit, it looks like a major answer is from /u/kieslowskifan, who adapts and updates his answer from time to time. The most recent version I can find is Why did Germany declare war on US on Dec. 11th, 1941. He lists a lot of lines of reasoning that the German command thought at the time.

In a sibling comment on the same posting, /u/bareback_cowboy cites and discusses Hitler's speech to the Reichstag, so that comment addresses the official reasons.

/u/dhpye commented in Would it have made a difference if Germany and Italy didn't declare war on the USA in WWII?, mostly concerning US war plans.

He wrote again in Why did Germany declare war on the USA when Japan attacked Pearl Harbour?, with more detail on the war plans. He ended with, "By declaring war himself, [Hitler] accomplished two goals: unrestricted warfare against US shipping within US coastal waters, and some level of personal satisfaction (I'm paraphrasing here, but he said something along the lines of 'Great Nations declare war upon others. They do not wait to have war declared upon them)'."