What’s driving Trump? Patriotism? Power? Revenge?

The key traits of this illness are a grandiose sense of self-importance; preoccupation with fantasies of unlimited success and power; a belief that he or she is “special”; a strong sense of entitlement; and a lack of empathy.

AMERICA, WHAT’S GOTTEN IN TO YOU? Understanding the Trump phenomenon

That’s one explanation for why Trump’s running. There’s another that goes back to 2011, when President Barack Obama tore shreds off the billionaire at the annual White House Correspondents’ Association Dinner.

Trump, who was then muttering about a presidential run but ultimately stood back for Mitt Romney, had been calling into doubt Obama’s right to be president, based on the discredited “birther” claims that he was not born in the US.

As Trump sat there with what the New York Times called a “frozen grimace”, Obama ridiculed Trump by comparing the decisions he took on his reality show, “The Apprentice”, with those of running the country.

“Now I know he’s taken some flak lately, but no one is happier, no one is prouder to put this birth certificate matter to rest than the Donald,” said Obama.

“That’s because he can finally get back to focusing on the issues that matter. Like, did we fake the moon landing? What really happened in Roswell? And where are (dead rappers) Biggie and Tupac?”

Trump was humiliated, according to the Times. His revenge was to seek the presidency.

It was also that Trump had raised millions for Romney but his public profile was so toxic that Romney’s campaign managers didn’t want him anywhere near Mitt; and most hurtful, he was denied a speaker’s spot at the 2012 Republican National Convention.

It can equally be argued that Trump’s 2016 campaign is revenge against the Republican establishment, which has never considered him one of their own.

Trump gave his own reasons to Time magazine last year, saying that he’d considered running in the past, but was busy with work.

This time, he said: “I didn’t want to look back in ten years and say I could have done that or I could have done that,” he said. “My family would look at me and say, ‘Ugh, stop.’ I had to do it for myself.”

The Wall Street Journal points out that Trump trademarked his “Make America Great Again!” campaign slogan in 2012, meaning his was planning his move back then.

It also argued that Trump, who first considered a bid in 2000, used his seasons on “The Apprentice” to build his personal brand and introduce himself to the American public.

Trump’s son, Eric, told a New York radio station in May that Trump’s final decision was not based on self-interest but international affairs, where his anti-diplomatic shoot-first, ask-later approach is of concern to the world.

Eric said his father’s deciding moment came when Obama did a deal with Iran to give it sanctions relief in exchange for reducing its uranium-enriching program.

“I think that was a game changer for him,” Eric said. “That is when he finally said, ‘Kids, I am going to it. I am going to give this a real shot.’”