But Trump finds Jong Un irresistible. “He wrote me beautiful letters. . . . We fell in love,” said the smitten Trump, who also described Jong Un as “smart,” “honorable,” “open,” “funny” and in possession of “a great personality.”

Another man, “Paul,” is a convicted felon, described by prosecutors as having “a hardened adherence to committing crimes and lack of remorse” after dabbling in tax fraud and money laundering.

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But the course of true love never did run smooth, and Trump fell for Paul, too. “Such respect for a brave man!” he said, calling him a “good man” even after Paul’s crimes became known.

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“Michael,” similarly, pleaded guilty of tax evasion, bank fraud, campaign-finance violations and lying to Congress.

But Trump, unaware that Michael would soon “flip,” lashed out at those who would destroy Michael “and his relationship with me.” Michael is “a fine person,” a “good person” and somebody “I have always liked & respected.”

Next, there’s “Roger,” who has been charged with lying about stolen emails. He just posted an image of the judge overseeing his case with crosshairs near her head.

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But Trump had fallen for Roger, too, calling him “a good guy” and “so loyal and so wonderful.”

Finally, there is a man — we’ll call him “Jinping” — who is holding 1 million people in concentration camps and whose dictatorship steals technology from Trump’s country.

But Trump tells Jinping, “My feeling toward you is an incredibly warm one. . . . There’s great chemistry.”

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To understand why Trump falls so hard for so many bad men, maybe we should take him at his word: It’s chemistry.

Evolutionary psychologists (not to mention Cosmopolitan magazine) have studied why some women love bad men. Research shows that men who possess the “Dark Triad” traits of narcissism, psychopathy and Machiavellianism are skilled at beginning relationships, deploying charm and manipulating would-be mates. Such men (few women have the Dark Triad) are more successful with women at first, before becoming abusive and unfaithful.

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Why a U.S. president would fall for bad men is less well known, given the small sample size. But something similar could explain Trump’s attraction to authoritarian and unsavory figures. If power is the ultimate aphrodisiac, as Henry Kissinger said, then absolute power is, like, wow.

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Gregory Carter, a psychologist at Britain’s York St. John University who studies the Dark Triad, explained to me why a leader might be attracted to a strongman of dubious morals. “They will use soft tactics — compliments, charm, praise — and hard tactics — bullying, threatening — interchangeably based on whatever will get the best result for themselves,” Carter explained. That’s beguiling stuff.

Sparks really fly when two Dark Triad leaders get together. “Individuals who are highly psychopathic gravitate to other individuals who are also highly psychopathic,” Carter told me. “Individuals possessed of this personality are relatively rare. So one who has defied the odds and become a successful psychopath represents something desirable” to a fellow psychopath, confirming “that this deceitful, personal, manipulative, callous perspective on the world does actually pay off.”

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Carter isn’t necessarily diagnosing Trump as a Dark Triad guy — which is good, because relationships between two Dark Triad people tend to be brief and explosive, even without nuclear arms.

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But Trump clearly crushes on bad men. He has spoken of his “great chemistry” not just with China’s Xi Jinping but also with Egypt’s Abdel Fatah al-Sissi , and he has a “positive chemistry” with Russia’s Vladimir Putin. At home, Trump’s fondness for bad boys is legend: Six associates have been charged in the special counsel’s probe, and four Cabinet officers have been driven out over questionable use of money.

Nobody enraptures Trump at the moment quite like North Korea’s Kim Jong Un. Kim “never had a relationship” like this before, Trump said before departing to meet the object of his affection in Hanoi. The recipient of gushing letters from Kim, Trump speaks of their “fantastic chemistry.”

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You can’t hurry love, though, and Trump now is in “no rush” to have North Korea denuclearize. Secretary of State Mike Pompeo, Trump’s wingman in Hanoi, must prevent the besotted president from dropping sanctions against Kim while getting nothing.

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Unrequited concession is a risk when you’re dealing with the Dark Triad. And Trump was already swooning when he landed in Vietnam on Tuesday. “So much love!” he tweeted. Even his Twitter account is aflutter.

Twitter: @Milbank