But in recent months, Mr. Bloomberg — who still owns 88 percent of the company — has become an increasing presence at Bloomberg’s Lexington Avenue headquarters. Those “few hours” soon turned into six and seven hours a day with Mr. Bloomberg taking a hands-on role in meetings and strategy decisions. Mr. Doctoroff, 56, a former deputy mayor of New York and private equity executive, told Mr. Bloomberg about two weeks ago that he planned to resign, frustrated with how the leadership dynamic had shifted. Mr. Bloomberg urged him to stay and remain chief executive, but Mr. Doctoroff demurred.

Mr. Doctoroff, who remains a friend of Mr. Bloomberg and will join the board of Mr. Bloomberg’s foundation, explained his decision to step down: “When Mike decided he wanted to spend some time at the company, and then spent more time, obviously things changed.” He added, “It isn’t the job I had for the past six years. It’s his — he wants to be involved. He doesn’t want to consult with me on everything. I get that.”

“This wasn’t the plan,” said Mr. Bloomberg, sitting next to Mr. Doctoroff on Wednesday at a coffee shop on the Upper East Side. “It was his idea. If it was up to me, he would have stayed.”

Mr. Bloomberg said he fell in love again with the company that he founded in 1981. He said that after vacationing for a couple of weeks in January and working on his philanthropy, he realized that he felt most excited by his work at Bloomberg L.P.

While Mr. Bloomberg said he did not plan to take the title of chief — “I hate titles,” he said — he intended to run the company for the foreseeable future. Still, he allowed that it was possible that he could hire a new chief executive if he were to decide to turn his attention elsewhere.