Dolly Parton met her husband, Carl Dean, at 18 years old on her first day in Nashville, according to her website. The music and entertainment icon had recently started her career when she ran into Dean outside the Wishy Washy Laundromat. The two dated for two years and got married in 1966. They were married 59 years, until his death in March 2025."Words can't do justice to the love we shared for over 60 years," Parton said in a statement after his death.Parton passed away on Tuesday after a brief battle with cancer.Over their many decades together, Parton was often asked about the secret to her and Dean's relationship success. It's a message that should resonate with couples seeking similarly lasting bonds. Here's what she said, and why experts think their relationship withstood the test of time.'Being opposite in some personality aspects can be an advantage'First, according to Parton, the two were opposites in many ways."He loves to go on vacation or on little trips, but he doesn't want to be gone a long time," she told People magazine in 2015. "He wants to be mostly around the house. He knows I'm just the opposite. I can't go enough places. I can't do enough things."Being different can strengthen a relationship — but only in certain ways. "A quiet person might be attracted to someone who is exceptionally outgoing and then social activities become easier as the talkative person does most of the work," says Amy Morin, psychotherapist and author of "13 Things Mentally Strong Couples Don't Do." If each partner's core values differ from the other's or they don't share a commitment to being together, however, keeping the relationship alive is hard, she says.'It's essential that each person has some independence'Parton always said the two were very independent. "He likes staying home, I like staying gone," she told Entertainment Tonight in 2006. "So that works out just fine for us.""It's essential that each person has some independence and gets some of their needs met outside the relationship," says Morin."When each partner has their own interests," she says, "they have different things to talk about. Independence can keep you interesting to one another." 'Having a shared sense of humor signals both cultural and intellectual compatibility'Dean made Parton laugh. "He has a great and warped sense of humor," she told People. "Research consistently highlights that humor compatibility is a major predictor of long-term relationship satisfaction," says Sara Nasserzadeh, author of award-winning book "Love by Design: 6 Ingredients to Build a Lifetime of Love.""Having a shared sense of humor signals both cultural and intellectual compatibility," she says. "When two people laugh at the same things, it shows they process information through similar cognitive lenses, understand the same nuances, and share a worldview regarding what is funny, what is absurd, and what is sacred." 'Don't try to change your partner'A strong relationship can create a base for success in the rest of one's life, too."A grounded, secure partnership frees up immense creative and professional energy, allowing someone to step into the world and accomplish extraordinary things," says Nasserzadeh. Dean might've been a key ingredient to Parton's success. "Don't try to change your partner," says Morin. "Support them in who they are and what they want to accomplish."