In fact if we agree that Trump tends to express admiration for authoritarian and nationalist leaders, and too often express disdain for democratic leaders, the likeliest explanation is simply that.

I think this is symptomatic of Trump's overly simplistic, 0-sum view of trade. It drives him to see our most important allies in peace since WW II, which are also our biggest trading partners (Canada, the EU, Japan) as foes. Ironically, maybe more than he sees authoritarians like Putin and Kim as foes. China is the exception - a global competitor who is also a very large trading partner - and not surprisingly, Trump is staking our a much more adversarial relationship with them than Putin or Kim.