A year ago, Theo Epstein was being serenaded by the delirious residents of a city, state and region, and promising the moon in return. Life was good. Epstein was just 30, having accomplished something that some men three times his age had waited a lifetime to see.

“To all the Red Sox fans,” he said beneath a champagne shower, moments after Boston swept the World Series in St. Louis, “we’ll do it again next year.”

Or not.

On Monday, Epstein walked away from what he described in happier moments as his dream job — even though he’d grown up in Boston and the offer he left on the table would have tripled his salary. He insisted everyone would be better off, and promised a more thorough explanation today. Absent any explosive new revelations, however, expect the old “philosophical differences” alibi to be backed out of the garage, dusted off and taken for a spin.

It’s not as though anybody believed Epstein and his mentor, Red Sox CEO Larry Lucchino, clashed about Descartes, although their arguments were over ideas even older — trust, envy, power — and almost as lofty. Still, the whiz-kid general manager is going to need another job someday, so don’t expect much detail. Lucchino, similarly, has kept a low profile, since the last thing he wants is to add fuel to the perception that he’s power hungry and a publicity hound, and as such, a difficult boss to work for.

Players who’ve spoken out have taken Epstein’s side, for all the good that’s going to do him.

“I’m very upset that we lost somebody like him,” Red Sox captain Jason Varitek said. “We seem to have a lot of that going around.”

Two weeks ago, right after Boston’s chance to repeat ended in the division series with a sweep by eventual Series winner Chicago, Epstein seemed to be focusing on the tasks ahead of him. Boston was facing the possibility of replacing three-fourths of its infield, two-thirds of its outfield and retooling both the starting staff and bullpen.

“You can’t be afraid of change,” he said. “You have to embrace it and you have to turn it into a positive.”

Sometime between then and now, events influenced Epstein enough to cut and run. The last straw supposedly came Sunday, with a column in the Boston Globe detailing how Lucchino bailed out Epstein in a failed trade, convincing the student that he could no longer trust his teacher.

Whatever the answer, an interesting counterpoint was provided late last week when Brian Cashman, the still-30-something Yankees GM who could have been Epstein’s rival for at least another decade, re-upped for three years with New York.

But he was also guaranteed much more of the torment that Epstein was so keen to avoid. Besides still-meddlesome owner George Steinbrenner, he’s shackled to an organization with more freelance advisers than the White House.

Cashman has survived eight years in New York, the longest stretch of any GM during Steinbrenner’s tenure, and somewhere along the way found out he loved the work too much to let go.

His life could be easier. He knows that. A year ago, just a few weeks before a reversal of fortunes launched the Red Sox to their first title since 1918, Cashman’s wife told New York magazine, “Brian would like to go to Boston and win the World Series as general manager of the Red Sox. That would be any man’s dream,” she added, “to go up there and become the god of Boston.”

Epstein had already achieved that, then decided it wasn’t quite enough. You have to wonder what the boy wonder is planning to do for an encore.