This is the only rational reason to join Canada to the US, and as a Canadian, I find it strangely compelling
It’s barely 25 years since we split them!
[Moderating]
The factual answer, such as it is, was given in the first four posts by @Der_Trihs and @Schnitte . Almost everything since then has been a better fit for P&E (as the OP acknowledged might be the case), so I’m moving this.
Our first form of a national union, the Articles of Confederation in 1777, included Article 11 stating that Canada could just choose to join the US Confederation as a state, without requiring any vote by the other states to accept them. (Any other area would require a 2/3rds vote of the existing states to allow that area to join.)
But 10 years later, when the US Constitution was written in 1787, that provision was dropped. It had become clear that the citizens of Canada did not want to join the USA (and US citizens in the other states weren’t sure they wanted Canadians to join unilaterally, either).
But Trump seems to have missed that.
I guess you can’t believe everything you read on the Internet. When you look at the actual text of Part V, Section 41 of the (Canadian) Constitution Act of 1982, which I did not look
at until this afternoon, this is what it had to say about requirements for unanimous consent:
Amendment by unanimous consent
41 An amendment to the Constitution of Canada in relation to the following matters may be made by proclamation issued by the Governor General under the Great Seal of Canada only where authorized by resolutions of the Senate and House of Commons and of the legislative assembly of each province:
(a) the office of the Queen, the Governor General and the Lieutenant Governor of a province;
(b) the right of a province to a number of members in the House of Commons not less than the number of Senators by which the province is entitled to be represented at the time this Part comes into force;
(c) subject to section 43, the use of the English or the French language;
(d) the composition of the Supreme Court of Canada; and
(e) an amendment to this Part.
So “unanimous” actually means the Senate, the House of Commons, and each provincial legislature. I still say this would be pretty tough.
That is, if Canadian law would even apply, which at least a couple of posters have said that it would not.
Historically States have sometimes been admitted in pairs to keep the partisan balance intact (Republican Alaska + Democratic Hawaii, for instance). So maybe Alberta can be the 51st State and the rest of Canada can be #52. (hypothetically, of course, I realize this actually isn’t, and shouldn’t be, happening).
What’s “if push comes to shove”?
The reality is that such a referendum would be a waste of time. In the case of, say, the Charlottetown Accord, there was significant public support for it. It didn’t pass but it was a reasonably close call.
A referendum to join the USA would fail by a margin of nine to one. That isn’t going to change. We can’t hold a referendum for every stupid idea.
How many people do you think live in Quebec? 
At the time, the parties were reversed.
How do you figure? Since Statehood, Alaska has voted Republican in every Presidential election except the 1964 LBJ landslide, and Hawaii has gone Democratic in every election except 1972 and 1984, also landslides.
That’s not how they thought things would go in 1959. For a long time Alaska had been largely Democratic (but not aligned with Dixiecrats), Hawaii largely Republican.
Within living memory, Newfoundland voted to stop being its own sovereign entity, and join Canada. So there’s precedent for this. If enough Canadians decide they want this, of course. That’s the sticking point. Even in the case of NFLD, there were a lot of people who didn’t want it. It’s worth reading the first part of John Crosbie’s autobiography to see how nasty that process was.
Didn’t we just get done with splitting them up?
Well, not “just”; it was done 26 years ago. But yeah, why re-merge them?
Unless the Senate changed its rules to require only a simple majority to shut off debate, or to set a time limit for it automatically shut off
Just to simplify things for the rest of the expanded United States.