my take: trump is obsessed with trade deficits. to his mind, trade deficits = others taking advantage of the US ("so unfair!"), AKA looking weak - something he won't tolerate. by selling more to the US than they buy, trading partners are "scamming" the US. the solution is to force those countries to buy more american stuff - you might recall that during his first term, trump shook down the chinese and they promised to buy more american soy beans, beef, cars, etc. (based on what i've read, they didn't actually order any of the stuff - but trump got to claim a win. pro-tip: when dealing with trump, let him have a big public win like a signing ceremony, then do nothing and quietly go back to how things were before)
when looking at the "problem" of deficits, i suspect that trump sees canada and mexico as low-hanging fruit: both countries are dependent on the US for their exports (something like 75% and 80%, respectively), so they should quickly buckle under US tariffs, right? they'll quickly cave in to US demands, order more made in USA, and trump becomes the savior of US industry.
unfortunately, whoever taught him about deficits must have skipped the lesson on interdependence. trump generally wants the US to be more inward-looking and protectionist, but that transition is at best costly when done right, and downright painful when done a la trump via a shock. of all the stupid shit trump has done so far, this by far has me the most concerned.
