John Cena and Nikki Bella's 'main cause' of split was over starting a family (exclusive)

John Cena and Nikki Bella have different ideas for their future.

After six years together, the pro wrestling couple announced on Sunday that they were splitting and calling off their wedding. Now, a source tells ET that the "main cause" of the breakup was over starting a family.

"Nikki and John truly love each other, but they will never have the same view on a perfect future," the source claims. "Nikki has always dreamt of having a family of her own and John does not want children and that was the main cause of their break up. They are both dedicated to their careers, but Nikki wants more than just that."

According to our source, Nikki has "fallen head-over-heels" for her niece, 11-month-old Birdie, the daughter of Nikki's sister, Brie Bella, and her husband, Daniel Bryan.

"[She] sees her sister's life and wants to have that too," the source notes. "Nikki thought at one point she could get past wanting children and that her love for John was enough, but as time went on, she has realized her dreams of a family would never happen with John. She couldn't get past the idea of not having her own baby."

See Nikki Bella and sister Brie:

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Nikki Bella and Brie Bella
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Nikki Bella and Brie Bella
LOS ANGELES, CA - NOVEMBER 13: TV Personalities Brie Bella (L) and Nikki Bella (R) attend NBCUniversal's press junket at Beauty & Essex on November 13, 2017 in Los Angeles, California. (Photo by Paul Archuleta/FilmMagic)
UNIVERSAL CITY, CA - OCTOBER 26: Nikki Bella (L) and Brie Bella visit 'Extra' at Universal Studios Hollywood on October 26, 2017 in Universal City, California. (Photo by Noel Vasquez/Getty Images)
LOS ANGELES, CA - FEBRUARY 17: Brie Bella (L) and Nikki Bella attend the 18th Annual Women's Image Awards at Skirball Cultural Center on February 17, 2017 in Los Angeles, California. (Photo by Rodin Eckenroth/FilmMagic)
LOS ANGELES, CA - AUGUST 13: Brie Bella (L) and Nikki Bella attend the Teen Choice Awards 2017 at Galen Center on August 13, 2017 in Los Angeles, California. (Photo by Kevin Mazur/Getty Images)
UNIVERSAL CITY, CA - OCTOBER 03: Brie Bella (L) and Nikki Bella visit 'Extra' at Universal Studios Hollywood on October 3, 2016 in Universal City, California. (Photo by Noel Vasquez/Getty Images)
INGLEWOOD, CA - JULY 31: WWE Divas Brie Bella (L) and Nikki Bella attend Teen Choice Awards 2016 at The Forum on July 31, 2016 in Inglewood, California. (Photo by Kevin Mazur/Fox/Getty Images for Fox)
WESTWOOD, CA - JULY 14: Professional wrestlers Nikki Bella and Brie Bella arrive at Nickelodeon Kids' Choice Sports Awards 2016 at UCLA's Pauley Pavilion on July 14, 2016 in Westwood, California. (Photo by Gregg DeGuire/WireImage)
NEW YORK, NY - MAY 16: (L-R) Nikki Bella, John Cena and Brie Bella attend the NBCUniversal 2016 Upfront Presentation on May 16, 2016 in New York, New York. (Photo by Slaven Vlasic/Getty Images)
NEW YORK, NY - DECEMBER 10: Nikki Bella and Brie Bella attend Diamonds Unleashed By Kara Ross Launch Party Hosted By Kara Ross, Anne Fulenwider, And Marie Claire Magazine on December 10, 2015 in New York City. (Photo by Monica Schipper/Getty Images for Diamonds Unleashed LLC)
PARIS, FRANCE - OCTOBER 08: WWE Divas Nikki Bella and Brie Bella pose during the 'Total Divas' Paris Photocall at La Maison Blanche on October 8, 2015 in Paris, France. (Photo by Kristy Sparow/WireImage)
NEW YORK, NY - OCTOBER 06: The Bella Twins Nikki Bella and Brie Bella are seen walking in Midtown on October 6, 2015 in New York City. (Photo by Raymond Hall/GC Images)
PASADENA, CA - JANUARY 14: Brie Bella (L) and Nikki Bella arrive for the 2016 Winter TCA Tour - NBCUniversal Press Tour at Langham Hotel on January 14, 2016 in Pasadena, California. (Photo by Gabriel Olsen/FilmMagic)
NEW YORK, NY - JUNE 24: Television personalities Nikki Bella and Brie Bella attend the 2015 NBC New York Summer Press Day at Four Seasons Hotel New York on June 24, 2015 in New York City. (Photo by Mireya Acierto/FilmMagic)
NBCUNIVERSAL CABLE ENTERTAINMENT UPFRONT -- '2015 NBCUniversal Cable Entertainment Upfront at the Javits Center in New York City on Thursday, May 14, 2015' -- Pictured: (l-r) Brie Bella, Nikki Bella, 'Total Divas' on E! Network-- (Photo by: Cindy Ord/NBCUniversal Cable Entertainment/NBCU Photo Bank via Getty Images)
NEW YORK, NY - MAY 15: Brie Bella (L) and Nikki Bella attend the 2014 NBCUniversal Cable Entertainment Upfronts at The Jacob K. Javits Convention Center on May 15, 2014 in New York City. (Photo by D Dipasupil/FilmMagic)
NORTH HOLLYWOOD, CA - FEBRUARY 10: WWE Divas Brie Bella (L) and Nikki Bella attend a 'Be A STAR' Bullying Prevention Rally presented by WWE And The Creative Coalition at James Madison Middle School on February 10, 2014 in North Hollywood, California. (Photo by Imeh Akpanudosen/Getty Images)
BEVERLY HILLS, CA - AUGUST 13: WWE Divas Nikki Bella (L) and Brie Bella of The Bella Twins attend the WWE SummerSlam Press Conference on August 13, 2013 at the Beverly Hills Hotel in Beverly Hills, California. (Photo by Barry King/FilmMagic)
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John has always been vocal about not wanting children, even opening up about it on Nikki and Brie's reality TV series, Total Bellas.

"Some people just don't want children. Everyone wants to blame John, but he feels he was honest and upfront with Nikki about never wanting to have children," the source says. "John's mistake was to then propose, because he knew that is what Nikki wanted most. He meant well, but in the end, it only hurt Nikki more and prolonged the inevitable."

"He began to make promises to Nikki that he just couldn't fulfill," the source adds. "Nikki wanted nothing more to believe that he had changed and he was trying but it wasn't enough. John felt as if he was really giving up part of himself by getting married. Bella felt she was giving up her opportunity to have children if she spent her life with John and eventually neither of them could get past it."

20 PHOTOS
John Cena and Nikki Bella together
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John Cena and Nikki Bella together
John Cena proposes to Nikki Bella during WrestleMania 33 on Sunday, April 2, 2017 at Camping World Stadium in Orlando, Fla. (Stephen M. Dowell/Orlando Sentinel/TNS via Getty Images)
2017 MTV Movie and TV Awards ? Photo Room ? Los Angeles, U.S., 07/05/2017 ? John Cena and Nikki Bella. REUTERS/Danny Moloshok
AUSTIN, TX - MARCH 13: Professional wrestlers John Cena and Nikki Bella attend 'Featured Speaker: John Cena' during 2017 SXSW Conference and Festivals at Austin Convention Center on March 13, 2017 in Austin, Texas. (Photo by Katrina Barber/Getty Images for SXSW)
John Cena proposes to Nikki Bella during WrestleMania 33 on Sunday, April 2, 2017 at Camping World Stadium in Orlando, Fla. (Stephen M. Dowell/Orlando Sentinel/TNS via Getty Images)
Cast member John Cena (L) and Nikki Bella attend the premiere of "Blockers" in Los Angeles, California, U.S. April 3, 2018. REUTERS/Phil McCarten
LOS ANGELES, CA - MARCH 11: John Cena, Nikki Bella arrives at the Nickelodeon's 2017 Kids' Choice Awards at USC Galen Center on March 11, 2017 in Los Angeles, California. (Photo by Steve Granitz/WireImage)
LOS ANGELES, CA - MARCH 11: Host John Cena (L) and professional wrestler Nikki Bella speak onstage at Nickelodeon's 2017 Kids' Choice Awards at USC Galen Center on March 11, 2017 in Los Angeles, California. (Photo by Frazer Harrison/KCA2017/Getty Images for Nickelodeon)
LOS ANGELES, CA - MARCH 11: John Cena, Nikki Bella arrives at the Nickelodeon's 2017 Kids' Choice Awards at USC Galen Center on March 11, 2017 in Los Angeles, California. (Photo by Steve Granitz/WireImage)
John Cena and Nikki Bella on stage at the 30th Annual Nickelodeon Kids' Choice Awards, March 11, 2017, at the Galen Center on the University of Southern California campus in Los Angeles. / AFP PHOTO / VALERIE MACON (Photo credit should read VALERIE MACON/AFP/Getty Images)
LOS ANGELES, CA - MARCH 11: Nikki Bella (L) and Host John Cena backstage at Nickelodeon's 2017 Kids' Choice Awards at USC Galen Center on March 11, 2017 in Los Angeles, California. (Photo by Alberto Rodriguez/KCA2017/Getty Images for Nickelodeon)
LOS ANGELES, CA - MARCH 11: Host John Cena (L) and professional wrestler Nikki Bella speak onstage at Nickelodeon's 2017 Kids' Choice Awards at USC Galen Center on March 11, 2017 in Los Angeles, California. (Photo by Kevin Winter/Getty Images)
LOS ANGELES, CA - MARCH 11: Professional wrestlers John Cena (L) and Nikki Bella at Nickelodeon's 2017 Kids' Choice Awards at USC Galen Center on March 11, 2017 in Los Angeles, California. (Photo by Alberto E. Rodriguez/Getty Images)
LOS ANGELES, CA - JULY 13: John Cena and Nikki Bella arrive at The 2016 ESPYS at Microsoft Theater on July 13, 2016 in Los Angeles, California. (Photo by Gregg DeGuire/WireImage)
NEW YORK, NY - MAY 16: John Cena and Nikki Bella attend the 2016 Fox Upfront at Wollman Rink, Central Park on May 16, 2016 in New York City. (Photo by Noam Galai/WireImage)
INGLEWOOD, CA - JULY 31: Host John Cena (L) and WWE Diva Nikki Bella attend Teen Choice Awards 2016 at The Forum on July 31, 2016 in Inglewood, California. (Photo by Kevin Mazur/Fox/Getty Images for Fox)
NEW YORK, NY - MAY 16:Nikki Bella, John Cena, Brie Bella at the NBC Universal Upfronts on May 16, 2016 at Rockefeller Center in New York City. (Photo by Steve Sands/Getty Images)
July 12, 2017; Los Angeles, CA, USA; WWE wrestler John Cena kisses Nikki Bella as they arrive for the 2017 ESPYS at Microsoft Theater. Mandatory Credit: Jayne Kamin-Oncea-USA TODAY Sports
Cast member John Cena (L) and Nikki Bella attend the premiere of "Daddy's Home 2" in Los Angeles, California, U.S. November 5, 2017. REUTERS/Phil McCarten
WESTWOOD, CA - APRIL 03: John Cena, Nikki Bella and Ike Barinholtz arrive for the Premiere Of Universal Pictures' 'Blockers' held at Regency Village Theatre on April 3, 2018 in Westwood, California. (Photo by Albert L. Ortega/Getty Images)
AUSTIN, TEXAS - MARCH 10: John Cena (L) and Nikki Bella attend the premiere of Blockers at the Paramount Theatre on March 10, 2018 in Austin, Texas. (Photo by Gary Miller/FilmMagic)
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Despite their differences, Nikki, 34, and John, 40, were very much in love, the source says, explaining that, "Their entire situation is actually heartbreaking."

"They had to split in order to give each other the life they both wanted," the source continues. "They truly supported each other's careers, but now Nikki wants more and I'm sure she will get it ... They both wanted to make each other happy, but eventually realized they needed to move on without each other for them both to have the life they deserved. They do love each other, but Nikki realizes now that love just isn't enough."

As ET previously reported, Nikki and John were just three weeks away from tying the knot on May 5 when they announced via social media that they were breaking up. "After much contemplation and six years of being together Nikki Bella and John Cena announced today their decision to separate as a couple," Nikki posted. "While this decision was a difficult one, we continue to have a great deal of love and respect for one another. We ask that you respect our privacy during this time in our lives.”

 

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