Now that Phelps is scheduled to end his 20-month retirement Thursday in Mesa, Ariz., a more nuanced question looms: Why?
Phelps sounded fairly certain he no longer wanted to confine his life to the pool when he walked away from the sport after the 2012 Olympics. He had seemingly put the perfect exclamation point on his career, winning gold in his last race to bring his record medal haul to 22, including 18 golds.
But several longtime Phelps observers say they see an athlete who's finally enjoying swimming on his own terms, without the pressure of outside expectations.
"I've never seen Michael this happy," says former Michigan coach Jon Urbancheck, who has known Phelps since the swimmer was 11 and recently visited with him in Baltimore. "This didn't come from his mom. It wasn't [coach] Bob [Bowman]. This was his decision."
Ian Crocker, one of Phelps' greatest historic rivals, wasn't surprised when he heard rumors of the impending return.
"Getting some time away, without the assumption he was coming back, was probably one of the best things for him mentally," Crocker says. "My hope is that he approaches it 100 percent on his own terms and just has his laughs with it. He's earned that."
After the London Games, Phelps immediately transitioned to a life many men his age would envy: playing golf and high-stakes poker, watching his favorite team, the Ravens, from luxury suites, dating beautiful women. He even made an acting cameo in his favorite television drama, "Suits."
So what drew the 28-year-old Phelps from his life of leisure back to the inglorious grind of daily practices at the Meadowbrook Aquatic Center in Mount Washington?
The immediate answer is that we don't know. Phelps hasn't commented since his return became official last week and doesn't plan to speak until Wednesday.
The only detailed comments from his camp have come from Bowman, his longtime coach, who has cast Phelps' comeback in simple terms — a master once more enjoying his art.
Bowman has brushed aside talk of the 2016 Olympics in Rio de Janeiro, insisting Phelps is merely testing his competitive form and has yet to design his path forward. If true, that's a distinct shift from the past, when Phelps and Bowman mapped years-long conquests of the sport's greatest records.
Great athletes end their retirements for any number of reasons. Some miss the adrenaline rush of competition. Some need the money. Some simply can't figure out what else to do with their lives.
Crocker says he has often felt the urge to return since retiring after the 2008 Olympics. "It's hard to resist chasing that thrill, once you've been on top of the mountain," he says, referring to both himself and Phelps.
Several people who saw Phelps at last year's world championships in Barcelona noted a restless fire in the swimmer's eyes as he watched the U.S. team compete without him. He wasn't used to being on the deck at a big meet, rather than in the water.
Urbancheck says Phelps also missed the familiar rhythms of a swimmer's life, the camaraderie of a world he had inhabited since he was a young boy.
"I think Michael likes having that structure in his life," the veteran coach says.
As the star of two comeback stories — one wildly successful, the other bittersweet — Michael Jordan is the modern king of the genre. The former NBA great was also Phelps' boyhood idol.
The swimmer has long elicited comparisons to Jordan, not just for his peerless excellence but for his ability to take perceived slights and turn them into fuel for his best performances. Even Phelps' proclivities for golf and Las Vegas card games seem taken from the Jordan playbook.