“Emotional incontinence is what sets Trump apart as a uniquely tyrannical figure. To watch him on stage is to witness a frenzied parade of inner consciousness. He’s simply incapable of restraining himself.”

That’s how I described Donald Trump in a rather feverish piece the day before the election. My concern then — and now — was Trump’s psychic volatility. No matter the stage or the subject, he always appears in the thrall of his emotions.

But Trump’s whimsical nature is compounded by an even worse instinct: domination. He has to win, he has to punish and humiliate. This is evident in Richard Branson’s account of his first encounter with Trump, which he described to Politico last year.

Branson, still in disbelief, recalled a lunch with Trump at Trump’s Manhattan apartment: “Even before the starters arrived he began telling me about how he had asked a number of people for help after his latest bankruptcy and how five of them were unwilling to help. He told me he was going to spend the rest of his life destroying these five people.”

Trump’s impetuousness flashed earlier this week when he attacked “Morning Joe” co-host Mika Brzezinski via an unhinged Tweet storm. The president called Mika “crazy” and “low IQ” and said she was “bleeding badly from a face-lift” when he saw her at New Year’s Eve party.

And why, you ask? Evidently Trump was irked by a joke Brzezinski made on air about Trump hanging a fake Time magazine cover on the wall of one of his golf club offices.

This isn’t surprising. This is who Trump has been for a long time. This impulse to attack was equally apparent during the campaign. You could see it in Trump’s vow to jail his political opponent; in his mocking of a disabled reporter who criticized him; and in his calls to “knock the crap” out of protesters. You see it today in his “running war” with the media.

Now that Trump has assumed power, the question of what drives him and why will become increasingly important.

Recently, I spoke with Michael D’Antonio, one of Trump’s biographers. I asked him for insights into Trump’s life and psyche: What drives him? Who influenced him? Where do his insecurities come from?

D’Antonio’s answers were instructive. If you want to understand Trump, he told me, look at the most important people in his life, the people who shaped his worldview. “He's always refused to be like other people when it comes to manners and respect for others and loyalty to the truth,” says D’Antonio. “I've always seen him as a man who defines himself by the number of norms he can violate. He's kind of a barbarian in that way — he gets a thrill out of disturbing other people and proving that he doesn't have to go along with what other people expect.”

According to D’Antonio, Trump’s norm-defying belligerence aligns with his heroes and the people he’s modeled his life around:

I think he's trying to be someone like he imagines Gen. Patton was. When he talks about these World War II generals portrayed in the movies, he's explaining something about himself and what he admires. He really does identify with that kind of aggression, that kind of authority. His role models were his father, the officer in charge of his group of students when he was student at a military academy, and Roy Cohn. All of these people were aggressive bullies who used strength as a tool and a measure of their own worth, and who believed anything is justified in pursuit of your ambitions.

Roy Cohn is an especially important figure. Cohn was a high-powered New York lawyer who made a name for himself as a legal hit man for Sen. Joe McCarthy in the 1950s. His brutality is the stuff of legend in New York circles. As this Washington Post report notes, Cohn had a “simple formula: attack, counterattack and never apologize.”

Next to his father, Cohn was arguably the most significant person in Trump’s life. As Trump came into his own as a real estate developer, Cohn was at his side, showing him the way. In a post for Politico last year, Michael Kruse quoted another Trump biographer, Wayne Barrett, who wrote in his 1992 book that Cohn “began in the mid-70s to assume a role in Donald’s life far transcending that of a lawyer. He became Donald’s mentor, his constant adviser on every significant aspect of his business and personal life.”

What Trump learned from Cohn was largely an extension of what he first gleaned from his father, Fred Trump. Fred was an intense, self-promoting patriarch who lorded over everyone in his orbit. I asked D’Antonio about the role Trump’s father played in his early life:

Well, his father told him, "You're a killer, you're a king," and expected him to be those things. His older brother Freddy, who died young, suffered for not living up the father's expectations that a man be incredibly tough and almost merciless. Donald was willing to be those things, and that's exactly who his father expected him to be. So he did achieve what his father expected of him. He is the person his father wanted him to be.

Efforts to explain Trump will no doubt fall short. Behavior is one thing, motivation another. We can see clearly what Trump does. Why he does it will always remain something of a mystery. But understanding the formative figures in his life offers as clear a glimpse as we can get.