"We literally spent a year basically doing therapy, working through some things, and figuring out what this next chapter would look like," Nick confessed.
"We've had to be really open and honest. There are things in the past I think that bothered us from time to time, even in our performances that we would do."
The 17 million album-selling band broke the hearts of legions of fans in 2013 when they broke up, right as they were preparing to release their fifth studio album, reportedly suffering a "deep rift over creative differences".
Nick admitted on Monday that he was nervous those differences still existed when they recently performed their first show together.
"I was so paranoid those first few two or three songs they would be mad at me after this, that I was not able to enjoy it," he recalled. "Then at a certain point I looked at them and thought, 'This is amazing. I'm going to have the best time ever.' And we did. A great show."
The boys stayed busy during their split; 26-year-old Nick maintained a strong solo career, which included his smash hit "Jealous", and just recently wed actress Priyanka Chopra. Joe, 29, launched the dance/rock act DNCE, and was equally successful with their trademark hit "Cake by the Ocean"; he also got engaged to Game Of Thrones star Sophie Turner.
Eldest Kevin, 31, meanwhile moved away from music for a time and into real estate development and app investment, started a family with wife Danielle; he is now the proud dad of daughters Alena, five, and Valentina, two.
Nick revealed on Monday night that they weren't even planning a reunion; rather they were filming a documentary for Amazon about their post-band lives, when something clicked.
"It was gonna be a story about where we are today," he explained. "And that was, separate lives doing separate things — Joe with DNCE, and me with my solo stuff."
"During that time, we started to realize there was a magic that we felt like we were missing. So the process was basically just, 'Why don't we give this another shot?'"
He continued: "We started playing a little music together and realized right away this is where we're meant to be in our lives at this moment."
Despite six years of maturing and one year of therapy, the brothers are still very much brothers: when asked by the host who was most likely to forget lyrics, Nick and Kevin immediately ratted the middle brother out.
To his credit, Joe admitted the same, and shamelessly confessed he now uses a teleprompter when performing live.