If tariffs are so obviously and universally bad, then why do tariffed countries respond with retaliatory tariffs instead of letting the first country shoot themselves in the foot and getting on with their own business?
In Canada's case the reliance of one trade partner (see https://atlas.hks.harvard.edu/explore/overtime/?exporter=cou... ) is probably a weakness that retaliatory tariffs can influence.
For example: if Donald Trump tariffs the country you are in charge of, you are likely willing to pay the economic cost of a retaliatory tariff to seem like you are standing up to America to your constituents. The politician’s goal is to be re-elected.
The politician may aim to deter getting tariffed in the first place, and promising a retaliatory tariff if another country tariffs you first is also reasonable as a deterrent from a game-theoretical perspective.
All it takes is for one world leader to not understand or care about the clear economic negatives to tariffs to start the chain: everyone else can be a rational actor.
Targeted tariffs can have more nuanced effects. You'll notice that Canadian politicians are talking about targeting red states with their tariffs. These tariffs will also economically hurt Canada, but you won't see a retaliatory tariff on things that hurt Canada disproportionately which would be the case with universal tariffs. You can expect them to pick luxury industries (like they did with bourbon) where preferences can be changed or industrial/commercial ones where (preferably) local or other suppliers are near parity with the US rather than say gasoline.
2) retaliatory tariffs are targeting the hostile actor, not widely against all (other) trading partners, thus redirecting trade towards less hostile country.
I imagine Carney is picking industries that are assymetricly painful to the USA or, since Trump isn’t up for re-election, congressional Republicans.