There is the theory that President Trump is still bitter about his Canadian hotel ventures that went bust.

Some, on social media, have speculated that a 2019 photograph in which Justin Trudeau appeared poised to kiss Melania Trump, the first lady, at a Group of 7 gathering in France, left Mr. Trump with a grudge against the dashing Canadian prime minister.

And then there is the transactional view, that Mr. Trump sees the acquisition of Canada as the 51st state as the ultimate real estate deal that would seal his presidential legacy.

As Mr. Trump prepares to push ahead with a new round of tariffs on the United States’ neighbors to the north and south, he has expressed a special brand of loathing for Canada. The bullying of a country whose most prominent stereotype is that its people are “nice” has led to political upheaval in Canada and created both consternation and speculation about why Mr. Trump wants to engage in a trade war with one of America’s biggest trading partners.

“I can’t quite figure it out,” said Stephen Moore, the Heritage Foundation economist and former adviser to Mr. Trump. “Whether it’s some kind of strategic leverage, I don’t know.”

Noting that there is “no love lost” between the president and Mr. Trudeau, Mr. Moore added: “With Trump, politics is personal.”