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:

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."

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.”