"Honeymoon phases end. They just do," the actor tells Glamour in its September issue, which features Olivia Wilde on the cover. "I think you have to find new elements that turn you on, and not only sexually."
Harris says starting a family with fiancé David Burtka rejuvenated their relationship.
"Having kids was one of those great moments for me. Watching David become another level of person, mastering this other domain, made me look at him with a whole other set of appreciative eyes. That made me re-fall in love with him," Harris, 41, says of becoming parents to twins Harper Grace and Gideon Scott, 3½.
"That's another important thing to realize. Everyone falls out of love with everything. You fall out of love with your house. You fall out of love with your job. You just have to figure out ways to keep [the love] alive."
Patrick Demarchelier / Glamour
Harris is not afraid to re-examine his beliefs. He says his Tony-winning role in Broadway's Hedwig and the Angry Inch encouraged him to re-think his concepts of confidence.
"I thought that masculinity equated confidence, and I was struggling to feel confident. Hedwig was a great active departure from that [mentality] because everything that I had known about confidence had to be thrown out the window," he tells the magazine.
"Now, all of a sudden, those masculine postures and inflections were tells that I wasn't doing the job right. I had to be overtly feminine, and it felt totally weird. Initially, walking around in heels is a very challenging thing for a guy to do. But once you've done it six or seven times, it's weirdly empowering. I had to move my hips and legs in a way that guys don't get to do so boldly – it was fun!"