The Aussie country star married the Hollywood superstar, with whom he has two daughters Sunday Rose, 11, and Faith Margaret, nine, in 2006.
However, he says that after nearly 14 years of marriage they still feel like high school sweethearts.
"With Nic, we have a passionate, curious and hungry togetherness," he tells British newspaper The Times. "She has infinite dimensions that keep her fascinating to me, and in the way we live our lives she could be my high school girlfriend. People give in to age too fast."
Talking about the secret to their healthy relationship, the 52 year old says that it's keeping their work lives separate while still inspiring each other.
"I have zero interest in acting. Nic has zero interest in being a musician," Keith reveals. "It makes for a harmonious flow in our house. But there's no question that Nic has been a huge influence on my creativity, simply for the way she approaches things in such a bold way. Curiosity makes her go toward something regardless of whether she thinks she can do it or not."
Nicole even inspired him to move away from country to a more R&B influenced sound on his 2013 album "Fuse".
"As a musician you can try anything that is of interest and I started doing that with Fuse, really as a result of seeing how Nic approaches things," he adds. "My dad would say, 'If you get asked if you can do something, say yes, then learn really quick.' Nic will agree to a project and then go, 'Oh s**t. Now I'm going to have to actually work out how to do this.' "