Trump fumed last week when FBI Director James Comey told senators that it made him "mildly nauseous" to think he tipped the balance in the 2016 campaign. A longtime Trump friend told CNN it made the President "white hot."

Another source close to the now-former FBI director said Trump fired Comey because he never provided the President with any assurance of personal loyalty.

That explanation tracks with the businessman-turned-politician's decades in the private sector, where he wrote and spoke repeatedly about the importance of loyalty, casting the trait as something every leader must demand from their employees.

Comey's firing has put that deep seated Trump desire on center stage, bringing to bear how the President's business history -- and management style -- are influencing the day-to-day operations of his White House.

During a question-and-answer session from The Learning Annex Wealth Expo, Trump was asked for the "key things" a boss should look for when hiring someone and building a team.

Trump was blunt.

"The thing that's most important to me is loyalty," Trump said. "You can't hire loyalty. I've had people over the years who I swore were loyal to me, and it turned out that they weren't. Then I've had people that I didn't have the same confidence in and turned out to be extremely loyal. So you never really know."

He added: "The thing I really look for though, over the longer term, is loyalty."

Read the books

This answer, along with the book he wrote with Trump in 2009, led Bill Zanker, the president and founder of The Learning Annex, to write: "Loyalty is important to Trump and is a wonderful trait to have in business."

Trump and Zanker worked together on "Think Big: Make It Happen in Business and Life," their 2009 how-to book on business that is chock full of Trump's musing about loyalty.

"I try to hire people who are honest and loyal. I value loyalty very much," they wrote.

"I put the people who are loyal to me on a high pedestal and take care of them very well," they wrote a few pages after. "I go out of my way for the people who were loyal to me in bad times."

Trump also uses the book to slam people who he views as disloyal, including contestants on his NBC show "The Apprentice," former employees and even former Democratic New York Gov. Mario Cuomo.

"Bill Clinton is a great guy with courage, Mario Cuomo is a disloyal guy without courage," Trump wrote.

Listen to the former employees

Former Trump employees and campaign aides, who requested anonymity to speak bluntly, told CNN that this isn't just mindless writing from the commander in chief. He truly craves loyalty and has for much of his career, they said.

One adviser said that this need is why Trump brought people like Keith Schiller, his longtime bodyguard and adviser, into the White House. Schiller, the adviser said, has many strengths, including his knowledge of Trump's wants and needs. But it is his unflinching loyalty to the president that cemented his role, they said.

Schiller's fidelity to Trump was on full display Tuesday, when the former New York Police Department detective hand-delivered the letter that informed the FBI the President was canning Comey.

Some of Trump's love of loyalty, another former employee said, stems from Trump's lawyer-turned-mentor, Roy Cohn.

Cohn, who first met Trump in 1973, was known for his toughness and ability to spin even the worst news into something that favored his client.

Cohn first represented Trump when the Justice Department accused the young real estate developer and his father, Fred Trump, of housing discrimination in 1973. Cohn came back with a $100 million countersuit and, two years later, the Trumps settled. That episode cemented Trump's relationship with the controversial figure and led Trump to cribbing some of Cohn's philosophies.

"Sometimes I think that next to loyalty, toughness was the most important thing in the world to him," Trump wrote of Cohn in his 1997 text "The Art of the Deal."

"He was a truly loyal guy -- it was a matter of honor with him," Trump wrote. "And because he was also very smart, he was a great guy to have on your side."