What's up with Donald Trump's fixation on Canada?

Is it just part of his "flood the zone" strategy? Or is he planning to turn the USA into an aggressor? Does he see himself as Emperor? Or are there actual economic reasons that I am failing to understand?

The US benefits from Canada selling its resources to them with a weaker currency. The "trade deficit" is more beneficial to the US than it is to Canada.

Canada joining the US as a state or even group of states would more likely than not make republicans unelectable (I know, I know... Canada would most likely just be a territory). And Americans are wealthier than Canadians so wouldn't this just lower the overall average?

I think about the constant threat of tariffs and I have a lot of trouble understanding what the motivation is. At the end of the day the US benefits greatly in being able to extract labour from Mexico, and resources (and labour to a lesser extent) from Canada. Obviously the current setup is mutually beneficial, but bringing production back to the US will just make American businesses less competitive internationally. Plus adding tariffs will hurt the Canadian dollar (and Mexican peso), thus offsetting the cost of tariffs for Americans to a certain degree.

I find it very difficult to find the economic justifications behind this. If companies facing tariffs were forced to move a lot of industry into the US they would most likely make use of AI & automation as much as possible, at a great short to medium term cost and probably not even generate significantly more jobs or even wealth for Americans in the long-term -- meanwhile there will always be international demand for Canada's resources.

What gives?