What really makes a relationship work? Young couples can get advice from the real experts: couples who have been happily married for decades.

Long-married couples have hard-won wisdom to offer, says Karl A. Pillemer, a Cornell University professor and the author of 30 Lessons for Loving: Advice From the Wisest Americans on Love, Relationships, and Marriage.

What can those people just starting out in relationships learn from those who’ve been in the trenches of romance? Here are a few lessons that might surprise you.

• Happy Couples Don’t Avoid Fights—They Pick Their Battles

Pillemer’s research reveals that people in long, successful marriage don’t avoid fights. They “have it out” over the issues that really matter while letting minor stuff slide. And they have the wisdom to know the difference.

Pillemer applied this advice himself, when he and his wife arrived at an impasse over a bathroom renovation. He wanted a stall shower; she wanted a clawfoot tub. Then he recalled the words of the 71-year-old April: “It’s important to let some things go, to figure out what matters and what really doesn’t matter.”

Knowing how important the tub was to his wife, Pillemer agreed to it. “It might sound small, but it was huge to us,” he says.

• Relationships Take Work, But So Does Everything
  

Older couples understand that marriages don’t get good and don’t stay good on their own, says Winifred Reilly, a licensed marriage and family therapist who has been married for 36 years.

Reilly recalls a fierce debate among readers of her blog, Speaking of Marriage. If a relationship requires too much effort, one argued, something’s wrong. The idea of working on a marriage struck her as dreary and tedious. “Why bother?” said the commenter.

“Why not bother?” Reilly counters. Most people understand that anything worth having takes effort. Staying fit takes daily exercise; a garden requires regular tending to thrive. “Love grows as much from the challenges we face and surmount together as from the delights we share,” Reilly says.

• After Romance Fades, Something Better Replaces It

Many young couples expect to feel “in love” forever, the same way they did when they first met, Reilly says. As a couples’ therapist, she often hears people telling their spouses: “I love you, but I’m not in love with you anymore.”

Baloney, Reilly says. Love changes over time, and that’s okay.

“That feeling of being ‘in love’ gets replaced by something that’s much more valuable,” she says.

Reilly believes that starry-eyed feeling is just nature’s way of getting relationships started. “If you quit when that fades, you are going to miss the best part—that deep, abiding friendship and intimacy that is much more sustaining,” she says.

Reilly adds: “The surprising fact about happy couples is that they aren’t always happy. At least not 24/7, jump-for-joy happy. In fact, the most successful couples I know will openly admit that they drive each other nuts.”

However, while romance may wane, sex often remains. Pillemer’s research indicates that many long-married couples continue to enjoy active sex lives well into their 70s and beyond.

“I don’t know whether young people want to hear that kind of thing,” one of his interviewees says. “They think when you get to have gray hair that sex just removes itself from your life, but that’s not true. Not at all.”

“There are lots of things to worry about in life,” Pillemer advises. “But fretting about sexless later years isn’t one of them.”