The Wall Street Journal interviews Steve Ballmer about his exit from Microsoft.

“Maybe I’m an emblem of an old era, and I have to move on,” the 57-year-old Mr. Ballmer says, pausing as his eyes well up. “As much as I love everything about what I’m doing,” he says, “the best way for Microsoft to enter a new era is a new leader who will accelerate change.”

Ballmer was not forced out according to the report.

The board’s beef was speed. The directors “didn’t push Steve to step down,” says Mr. Thompson, a longtime technology executive who heads the board’s CEO-search committee, “but we were pushing him damn hard to go faster.”

Investors, too, were pushing for transformation. “At this critical juncture, Wall Street wants new blood to bring fundamental change,” says Brent Thill, a longtime Microsoft analyst at UBS AG. “Steve was a phenomenal leader who racked up profits and market share in the commercial business, but the new CEO must innovate in areas Steve missed—phone, tablet, Internet services, even wearables.”

Ballmer was convinced that he had to change the company culture.

Qi Lu, an executive vice president, submitted a 56-page report on applications and services. Mr. Ballmer sent it back, insisting on just three pages—part of a new mandate to encourage the simplicity needed for collaboration. Mr. Lu says he retorted: “But you always want the data and detail!”

Mr. Ballmer says he started to realize he had trained managers to see the trees, not the forest, and that many weren’t going to take his new mandates to heart.

“At the end of the day, we need to break a pattern,” he says. “Face it: I’m a pattern.”

Ballmer decided to step down.

At the board’s June meeting in Bellevue, Wash., Mr. Ballmer says he told the directors: “While I would like to stay here a few more years, it doesn’t make sense for me to start the transformation and for someone else to come in during the middle.”

So far, this decision has been positive for Microsoft and Ballmer, whose Microsoft shares appreciated by $1.7 billion since announcing his retirement.