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 Most posts on this subject have been archived, so you can't ask followups there.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. In another comment, /u/bareback_cowboy cites and discusses Hitler's speech to the Reichstag, so their comment covers the official reasons.

One possibility not mentioned there is that the US war plans were leaked a few days before Pearl Harbor. That was mentioned by /u/dhpye in Would it have made a difference if Germany and Italy didn't declare war on the USA in WWII?. He suggests that Germany considered that the leaked plan had the US sending in troops only in 18-30 months, during which time Germany might well win the war anyway. A reply from him elsewhere summarizes it as "By declaring war himself, he 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)'."