If you want to know how to keep your car running, ask a mechanic. If you want the lowdown on caring for your pipes, call your plumber. Why? Because people who deal with things when they're broken often know the most about how to keep them in good repair. James J. Sexton thinks the same principle applies to divorce lawyers.

In his new book, If You're in My Office, It's Already Too Late, the 20-year veteran of every kind of divorce imaginable dishes out advice on how to avoid needing his services based on his up-close experience of marital breakdown. It might not be the first place you'd look for relationship advice, but when you think about it, it makes a lot of sense.

And when you read a recent interview with Sexton in Vice it makes even more sense. In a long chat with Sean Illing, Sexton offers incredible insights into why marriages fail (social media is nearly always involved these days, he reports) and what that says about how to keep them strong. The whole piece is well worth a read in full but much of Sexton's wisdom can be boiled down to just four insightful words.

We fall out of love "very slowly, then all at once." 

If you asked people to name common reasons for couples to get divorced, you'd probably hear a lot about dramatic conflicts -- infidelity, financial disagreements, sexual mismatch, differing visions for the future. And Sexton confirms that these sorts of big, gnarly issues are indeed the immediate reason most people find themselves in his office. But he insists they aren't the real reason marriages break down.

"From my perspective, these big reasons have their origins in a succession of smaller choices that people make that take them further and further away from each other," Sexton says, adding:

"In Tom Wolfe's Bonfire of the Vanities, one of the characters is talking about how he went financially bankrupt and one of the other characters says, 'Tim, how did you go bankrupt?' He said, 'Well, I went bankrupt the way that everyone does, very slowly and then all at once.' I think that's how marriages end. Very slowly and then all at once. There are lots of little things that happen and then the flood comes."

What sort of "little things" does he mean? "That annoyed look on your face, that time you ignored your partner when they needed you, all those times you couldn't bother to give that person your full attention. These are the small things that become big things over time," he offers later in the interview.

"Love is a verb."

This is a beautiful description of how relationships work that accords with how so many big changes in life really happen. Success doesn't happen overnight, neither does physical fitness, and neither does relationship breakdown. Nearly all big goals come from weathering small setbacks and making steady forward progress over days and months and years. And then there's often one big breakthrough. Outsiders only see the dramatic final stages of the process, but the roots are generally deep.

The best part of Sexton's interview isn't his explanation of how marriages decay, however. It's the solution he offers couples. It's only four words long, so no one has any excuse for forgetting it: "Love is a verb."

"I'm a romantic, but I don't believe in fairy tales. I think that we sell people a bill of goods about what love is supposed to look like. Love is a verb," he insists. "Falling out of love is very slow. It's a very gradual process. You put on weight slowly. ... You don't just wake up one day and you've gained 20 pounds. You very slowly gain weight, but sure enough, it happens. It's the same thing with love."  

And not falling out of love, like not gaining weight, isn't about dramatic gestures or heroic acts. It's about a relentless daily commitment to small actions. It's about doing things -- not clamming up to avoid the fight, not complaining about how the towels are folded, reaching out a hand in a tense conversation. In other words, it's a verb.

"If you want to keep your love alive, you have to be attentive to all the little things that go wrong along the way, and constantly course-correct. If you can do that, you'll never set foot in my office," Sexton concludes.

You're unlikely to hear better marriage advice anytime soon.