1. The U.S. economy is large, but insular. Most trade is between states, not with other countries. This is why most countries shrug and accept tariffs on their goods. For them, trade with the U.S. is not as big of a deal as it's portrayed, so it's not worth risking "fireworks" over. Canada and Mexico are rare exceptions to this. Canada, particularly, is reliant on international trade and trade with the U.S.. That's why they are fighting back for a deal that's better than slow death.
2. Canada is the #1 export market for a lot of American goods. e.g. Autos. Trump is determined to destroy Canada's auto manufacturing sector and bring it all to the U.S.. That might sound good to Americans, but they need to realize that this will necessarily mean they'll lose their #1 export market. If tariff's freeze Canada out of the American market and the Canadian auto industry dies, there will be no incentive to maintain the trade barriers against Chinese autos. Thanks to heavy automation, China is producing better autos than North America can at a fraction of the cost. American autos will be more expensive than ever after a forced retreat from Mexico and Canada and will be totally unable to compete on price and quality. Then there will be the lasting hostility to American products that Trump has caused.
3. Americans should closely examine some of the demands that were in the deal Canada just walked away from. One was for Canada to follow U.S. trade policy with other countries, no questions asked, no variations allowed. e.g. If the U.S. decides Tuvalu needs to be tariffed, Canada would be obligated to as well. Canada would also need to ask permission to sign trade deals with other countries. This would have been a complete surrender of sovereignty and would have reduced Canada's international trade policy to that of an American vassal. Not even an American state! This was a completely unacceptable proposal that was likely introduced to sabotage the negotiations. Howard Lutnick's last minute arrival and direct financial interest in generating more tariff refunds is not getting enough attention.
4. Americans are mostly ignorant of the fact that one of the biggest trade wars of the last century is going on with their Northern neighbour. Of those who are aware, the overwhelming majority are not supportive. Canadians are universally aware of it and overwhelmingly in support of Carney. Canadians have been cutting travel to the U.S. and buying fewer American goods for over a year. Now they're going to do it harder, and many won't stop even after the trade-war ends. Kids are watching their parents read labels in the grocery store to make sure they're not buying American goods and asking why. Trump can negotiate a new trade deal with Carney but he can't negotiate away the sense of betrayal Canadians now feel.
5. People are saying that Canada is too small to win this trade war, but this is not even remotely true. In Canada the war is supported because it's a matter of sovereignty, which is priceless. In the U.S., it's fracturing the Republican party. In Canada, people are voluntarily boycotting American goods and services, as well as travel to the U.S.. In the U.S., there are looming mid-terms that rising costs from a fresh trade-war could help decide. The most recent tariffs imposed by Trump were likely aimed at forcing an agreement he could tout as a "huge victory" right when he needed it. Instead, he has an embarrassing fiasco blowing up in his face. He's so short on ideas that he's threatening to rename lakes! The U.S. may be big, but U.S. leadership is small, inept, and poised to lose it's hold on power. Carney has signalled that he's ready to maintain this trade-war for the two years and change needed for Trump to leave office, but a few months may be all that's required.