Mattis is the only relevant quote here. He sees them as a threat because they don't comply with NATO. He also respects Trump's ways of getting the best defense per dollar, apparently, which is news to me. Views on NATO are outdated, we should have threatened to drop it three decades ago. Even Mattis has been quoted:
"February 15th America’s defence secretary, James Mattis, warned allies that they must adopt plans to raise their military spending or risk seeing its most powerful member “moderate its commitment to the alliance”."
NATO members that aren't the US absolutely do not pay their fair share, and we shouldn't cater to that.