Trump attacks Canada that because he's a rich narcissist spoiled manchild who spent his entire lifetime getting what he wants and surrounded himself with people who will tend to every one of his caprices and Canada dared to say "no" when he demanded the country to be turned over to him.

He is a freak of nature of a combination of pure shamelessness, narcissism, confident incompetence, ability to lie like he breathes and megalomania. It is statistically improbable that whatever shitbag replaces him will be anywhere nearly as bad as him. I expect his replacement to do nothing less than immediately try to mend relations with Canada because they will want to fix their country's economy.