Image copyright Getty Images

We've probably all thought about that day, no matter how far in the future, when we can finally pack it all in and retire. No more early alarms, no more being told what to do - time to spend doing exactly what you want to do.

So why do so many famous retirees choose to step back into the limelight? The obvious answer is money - and with Floyd "Money" Mayweather announcing he's "coming out of retirement in 2020", some might think he's clocked a way to add more zeroes to his bank account.

But there could be something else that causes famous sportsmen and celebrities like Jay-Z and Emma Watson to leave their retirement dreams behind.

"Retirement is actually one of the most stressful life events, believe it or not," says Dr Punit Shah from the University of Bath's psychology department. "It's in the top 10 or 20 of life's most stressful events."

A lack of routine

Routine is important to a lot of people, but it can be especially so for sports people - who've spent their life following a rigid schedule to keep them in peak physical and mental condition.

"That basic routine is disrupted by retirement, which is actually quite a psychologically difficult thing to deal with," Dr Shah says.

"When you're a sports person in particular, and especially a highly-prized sports person, often why you're as good as you are is because you're able to maintain that discipline and routine.

"So that disruption is even worse for people that are so highly trained."

You might think that's not such a big deal. Surely Floyd Mayweather - and other sports stars who've retired only to return, like Michael Phelps and Michael Jordan - have enough people around them to help start a new routine doing something different?

But Dr Shah says it has a knock-on effect.

Image copyright Getty Images Image caption Floyd Mayweather - now 42 - has been boxing professionally since 1996

"The sort of extension of that is, we build an identity around our work and it's that identity which you lose.

"You can go from being the most vicious, agile, exciting boxer to a dad or a granddad. That shifting identity is really quite difficult for a lot of people to handle."

Tied into that could also be a "fear of irrelevance".

"That fear of losing yourself, losing the sense of self - and losing your identity. It's that sort of, 'Oh no, I've been forgotten about. No-one cares about the Spice Girls, I'll come out of retirement' - all of a sudden I might be back in back in the headlines."

Another reason someone might choose to come out of retirement is that according to some studies it can "lead to quite negative health consequences".

Image copyright Getty Images Image caption The Spice Girls went on a successful reunion tour in 2019

"The psychology here is perhaps unclear, but the rates of heart attacks, strokes and other illnesses sometimes goes up in people that retire early or after retirement."

Dr Shah stresses that we don't yet really understand the reasons behind this, but speculates it comes back to that changing routine.

"People coming out of retirement, and particularly when you're in peak physical shape and you're suddenly feeling really terrible, you can see why people would not even want to but need to get back into it, to maintain their physical health. It may well for some people be the best thing to prevent serious illness."

The elephant in the room

Image copyright Getty Images Image caption Floyd bloody loves money - here he is throwing some of it at Conor McGregor

We've got through a whole article about Floyd Mayweather without seriously interrogating the financial aspect to this story, which might be a first.

Mayweather has come out of retirement twice in the past few years - first to face Conor McGregor in a boxing match that may have been worth up to £230m, and then to face Japanese kickboxer Tenshin Nasukawa - bagging a reported £7m for just 140 seconds of work.

So you can see why a potential fight in the UFC would be an interesting proposition. It's rumoured that's where he's headed after UFC president, Dana White, reposted Mayweather's Instagram announcement.

Dr Shah says the financial incentive for athletes and pop stars to come out of retirement could definitely play a part - even when, like Floyd, your bank account is already looking pretty stacked.

"It's not even that they need the money, often these guys are very wealthy.

"And the reason why they get back into it is not just for the money - it's part of the game of making money and getting that attention."

Dr Shah adds: "There isn't evidence for this, but it certainly seems seems like a reasonable theory."

Follow Newsbeat on Instagram, Facebook, Twitter and YouTube.

Listen to Newsbeat live at 12:45 and 17:45 weekdays - or listen back here.