It's the talk of the wrestling world that John Cena broke bad and joined up with The Rock at Elimination Chamber. The real question people should be asking, though, is why John Cena turned heel anyway.
Cena just earned a WWE title shot by winning the chamber and only turned on Rhodes after Cody turned down an offer from the Final Boss. The Road to WrestleMania will likely round out the story for Cena vs Rhodes, but for now, there are many story possibilities to explain the turn.
Cody Rhodes can't hack it at the top
Last year, The Rock and Cody Rhodes was clearly the direction for WrestleMania in Las Vegas. The rationale, beyond the Bloodline beef, looked to be Rock wondering aloud if Cody could carry the title with honor. Cody's title run has been a little uneven. The story of a new champion struggling with the pressure looked plausible. When it became clear Rock wasn't the direction, that potential feud started to fade. Then John Cena got involved.
Cena has held more titles than The Rock, than anyone in wrestling, save for Ric Flair. He carried the company longer than any top star in the promotion's history. John Cena agreeing with The Rock and siding with him to get the title off a rookie "QB1" and onto the most successful guy could be interesting. Also, his being the man benefiting from this shift would make a heel turn not simply justified but required, as it would make the decision inherently selfish, even as Cena doesn't admit to that ever.
Jey Uso is why John Cena turned heel
A cadre of internet fans has declared Jey Uso insufficiently talented or popular enough to have a world title match at WrestleMania. Thus, his Royal Rumble win is a travesty. It would stand to reason that losing to such an inferior opponent could provide context for why John Cena turned heel. As a 16-time champion, Cena has the pedigree Jey supposedly lacks.
The Jey Uso backlash is excessive, but the best heels have a kernel of truth to them. If there is a strand of people who view Uso as inferior, then having that loss is what made Cena align with The Rock makes sense. Jey may lose to Gunther. It's also possible that Cena will lose to Cody Rhodes. As long as one of them leaves WrestleMania as a world champion, the two of them can feud; WWE's hottest star today and its biggest star ever fighting for the gold means Cena will have helped two current WWE talents get over.
John Cena's time is (still) now
John Cena was involved in a feud for the WWE title against Mark Henry, which kicked off with an incredible segment and a faux-retirement angle. Cena's retirement tour, which kicked off in earnest at Royal Rumble, was different. It was clear he meant it when he said he was done, and why wouldn't he? Cena never lies; it's what he is best known for as a WWE superstar. The bait and switch he fell victim to was beneath the GOAT.
After a turn that shocked everyone and clearly aligned with The Rock, who isn't going anywhere, Cena's statements about leaving may be part of a bigger story that explains his turn. It may also signal that he is staying a little longer. On his own, he is an aging superstar on his last run. With the backing of TKO, he can extend his career. Having friends in high places means limited dates, lots of opportunities, etc. A story where Cena isn't willing to let it go could be very interesting. It could also have some enjoyable endings.
Why else might John Cena have turned heel?
There are a lot of business reasons for the Cena turn. It solidifies Cody as the face of WWE, generates a lot of buzz, and sets a CM Punk vs. John Cena retirement match that could be the greatest feud in WWE history if done right. All of these are true, and they each require a solid and compelling explanation for why John Cena turned heel.
Fan interest has not been this high for a John Cena program in a decade. WrestleMania 41 is starting to heat up right as fans were getting worried. Cena, like The Rocky last year, has injected some star power and powerful character into the build, a seemingly crucial hallmark of the Major PLE booking under Triple H. Until we learn more, Cena's decision will remain a compelling enigma best summarized as this; for the first time in a long time, John Cena is a bad, bad man.