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.
Athletes in all sports will tell you fans have it wrong when they measure careers in the language of milestones and tidy narratives. The internal focus is more on winning the next race or game, mastering the next skill. It's how great athletes got great in the first place, and most don't flip off the switch simply because they achieve tremendous goals.
From the outside, perhaps, Jordan authored the perfect ending when he hit the series-winning shot in the 1998 NBA Finals. But that didn't mean the ending felt perfect to Jordan, who lived for competition and believed he could still play effectively three years later.
Some would argue Jordan tarnished his legacy by returning as a merely excellent guard in the unfamiliar jersey of the deeply mediocre Washington Wizards. Jordan has never expressed any such regrets.
Orioles great Jim Palmer attempted a comeback at age 45, the year after he'd been inducted to the Hall of Fame. He says he did it because he honestly believed he could help the Orioles. Palmer ended his quest after a rough spring training start against the Boston Red Sox, but he says he never cared if others judged the comeback a failure.
Palmer says the process is often an internal one, driven by the athlete's questions about what he has left.
"Life's about setting goals and meeting challenges," he says. Phelps "knows what he has to do. Who better to test whether he can still do it?"
Swimming commentators have scoffed at the notion Phelps might diminish his career by swimming at sub-peak form. With less than two years away from competition, he's hardly a dinosaur, they say. Comparisons to old rival Ian Thorpe, who failed to make the Australian Olympic team in his 2012 comeback bid, are inevitable. But Thorpe had been away nearly five years.
The list of post-30 Olympic champions (Phelps would be 31 in Rio) is short. But it's longer than the list of swimmers who have dominated the sport like Phelps. He has always thrived on reaching for the unlikely.
"My first inclination is not to put any kind of limit on what he can do," says Phelps' former Olympic teammate Aaron Peirsol, who will be on hand to lead a youth clinic at the meet in Mesa.
Regardless of how Phelps performs, the swimming world seems excited to have him back. He's perhaps the only swimmer who could double ticket sales (as organizers say he has in Arizona) or generate national headlines, just by announcing he'd compete again.
There's no indication money is a major motivator for Phelps' comeback. But if he competes in Rio, he will extend his shelf life as a big-ticket endorser and ambassador for swimming, says Bob Dorfman, creative director at Baker Street Advertising in San Francisco.
"It obviously makes a lot of difference when you're still top of the mind with consumers," Dorfman says. "It could be worth millions of dollars to him to compete."
Phelps' presence would also guarantee greater attention on swimming, an appealing notion for an athlete who has always said he wants to change the sport.
A looser approach
It's easy to forget, given the four gold medals he won in London, but Phelps' commitment to swimming wavered in the years leading up to the 2012 Olympics.
He talked of walking away in 2009 because of the intense scrutiny he endured when a photo surfaced of him lifting a marijuana pipe to his lips. A once-relentless trainer, Phelps barely practiced some weeks, leaving Bowman frustrated and concerned that his star pupil might be headed for an Olympic embarrassment.
When he announced he was done after the London Games, Phelps promised the world would never see him swimming at age 30. He sounded relieved to have the toil and pressure behind him.
Even last summer, as rumors of a comeback first bubbled, Phelps and Bowman downplayed the possibility. The swimmer said he was enjoying life too much. The coach said he wouldn't even condone a comeback if Phelps seemed haphazardly committed.
Thus Bowman has said he was surprised when Phelps called last fall, wanting to resume training. The swimmer also rejoined the drug testing pool for the U.S. Anti-Doping Agency, a sign he was at least contemplating a return to competition.
Bowman has said Phelps is training five days a week, less than at his peak but enough so he likely won't embarrass himself in Mesa. The coach has emphasized how much fun the record-setting Olympian is having, especially compared to his pre-London training.
Bowman has also made it clear Phelps isn't trying to be his old self. He's aiming to swim only shorter events — the 100-meter butterfly, 100-meter freestyle and 50-meter freestyle in Mesa — leaving behind the grueling 400 individual medley. He has even talked of using a butterfly stroke in the 50-meter freestyle as a fun experiment.
Will this looser approach make a run at a fifth Olympics more palatable? Time will tell, with Phelps entered in three Grand Prix events between now and the end of June and perhaps eyeing the U.S. national championships in August.
"I doubt it's as simple as 'I want to win another gold medal,' " Peirsol says in assessing Phelps' motivations. "It's more visceral, more internal. But he doesn't owe anybody any explanations."