A good pimp knows that when one of his hoes gets uppity, the threat of sanctions is often enough to elicit the proper attitude adjustment. Trump is a good pimp, and Canada is one of his hoes.
Trump doesn't want to impose tariffs on Canada; it will hurt the economies of both Countries, particularly if Canada reciprocates. He does, however, want Canada to tighten border security, and he wants Her to pay Her fair share of mutual military expenditures. Threatening to impose tariffs is a pressure tactic designed to force Canada to act, because asking nicely doesn't work.
If you judge Trump by his results in this term, as opposed to fixating on his methods, I believe that he will achieve a lot of positives by the end of his current mandate. The people who won't be happy will mostly be those who complain the most and contribute the least. They currently aren't a large enough minority to collectively hold the balance of power, and Trump's attitude towards them is "Fuck 'em".
Consider the high school gym teacher of old, the football coach, or the drill sergeant in army boot camp. They frequently use/ used humiliation as a motivational tool. They are generally despised when they are doing their job, and they expect that. When their job is complete, however, many of their subordinates acknowledge that success would not have been accomplished, if they had been politely asked to do what was demanded of them.