Cena said he was going through a difficult period about four years ago and started writing in a journal to help him cope. He later started sharing the journal entries on Twitter.
"It was about four years ago. And I decided just to have a vulnerable moment and put one of those thought-provoking journal entries out there on Twitter," Cena recalled.
Cena said many of the journal entries were about self-love as he struggled with feelings of being not enough. The entries coincided with the message of BTS' 2017 EP Love Yourself: Her.
"So I put out all these messages about self-love. And I'm thinking, this is not going to be a good idea because this big, larger than life, sixteen-time WWE champion is is now talking about vulnerability and the fact that you are enough, and you shouldn't be ashamed of who you are. But it caught fire because of the BTS Army," Cena said. "I was essentially sending a similar message of the band."
Cena shares his journal entries and a message of self love in the inspirational books Do Your Best Every Day To Do Your Best Every Day and Be a Work in Progress.
"These books exist because the Army was brave enough -- the BTS Army was brave enough to support my vulnerable moment. And now it's become a part of my daily routine," Cena said on Ellen.
"It's basically my journal," he added. "I contemplate how I'm feeling. I basically try to put forth a thought. I never try to squeeze it to a certain individual. I try to be as inclusive as I can."
"I got interested in their music, and then I got interested in what the music stood for," he said. "They advocate self-love. They advocate don't be afraid of failure. They advocate that you are enough."
Cena will appear in the upcoming films F9 and The Suicide Squad.