Time flies when you’re madly in love.

Later this year, Blake Shelton and Gwen Stefani will be celebrating their tenth anniversary together — and the country superstar tells PEOPLE exclusively in its latest issue that they’re more in sync than ever.

“It honestly does [feel like time is flying by]. Some of the things that we talk about, we’re to the point in our relationship that it’s like, ‘Oh, remember what happened…’ and you realize, ‘Oh my God, that was 8 years ago!’ It’s like, how did this happen so quickly?” says Shelton, 48.

But while Shelton acknowledges that “10 years is a long time,” the singer — who teamed up with Stefani for a new duet on his latest album For Recreational Use Only (out May 9) — says the relationship “still feels new for me.”

“I feel like that might be the key to happiness, is that it feels just as exciting and new and happy,” he says.

While only one track made the album, Shelton reveals he and Stefani, 55, actually cut two duets. “We were both so fired up about both of them that we ended up holding one back for the next project,” he says.

“Hangin’ On” was “definitely a challenge” to record, Shelton adds, noting that he and Stefani “love to push ourselves as vocalists.”

“We live together, so we walk around the house singing these songs all the time and we have months to talk about, ‘Hey, maybe you jump on that part,'” explains Shelton of their process. “By the time we get to the studio, we’re normally really prepared.”

Related Stories

Gwen Stefani and Blake Shelton’s Relationship Timeline

How Blake Shelton Overcame Burnout After The Voice (Exclusive)

Stefani has long been a vulnerable songwriter, pouring her emotions into lyrics — and Shelton says she remains just as passionate about writing decades into her career.

“Gwen is way more particular when it comes to a lyric. I’ve never been as much of a stickler as she is. It’s really important for her to be able to see herself in the lyric that she’s singing vs. me. I’ve sung about going to prison, and there’s never really been a debate,” he jokes.

And while he’s busy promoting his new music, the star says at the end of the day he’s looking forward to a slower summer with his family, including stepsons Kingston, 18, Zuma, 16, and Apollo, 11.

“I’m looking forward to the kids being out of school. Gwen and I pretty much have our summers wide open. We both have a few shows here and there, but nothing that’s tying us down to keep us from going and doing something fun,” he says. “We don’t have any plans — which is our favorite plan.”