It first came out--of course--on Page Six. Robert De Niro and his wife Grace Hightower have split up. No big surprise, you might think--celebrity marriages fail all the time. But this one had lasted more than 20 years. In fact, it's survived so much that you might have assumed they were together for life. But that's the lesson here: No matter how long you've been together, no matter how fine you think things are, a marriage or partnership is never, ever a done deal.

De Niro and Hightower certainly didn't rush into things. The two met when Hightower, a former flight attendant, was working as a hostess at the high-end restaurant Mr. Chow, popular with Hollywood celebrities.

They dated for a decade before tying the knot. "It was an ease-in. It wasn't a whirlwind," Hightower told The New York Times. Almost exactly nine months after they wed, their son Elliot was born. Then, in 1999, they split up, De Niro filed for divorce, and there was a custody battle for Elliot. But instead of going through with their divorce, the couple reconciled and even renewed their vows at a star-studded ceremony in 2004. In the meantime, Elliot was diagnosed with autism, a heartbreaking event for both parents. The couple had their second child, Helen Grace, by surrogate, in 2012. 

In other words, theirs is a 30-year relationship that has survived a lot of bad times. He's 75. She's 63. You would think they'd have figured out by now how to make a marriage work. And, according to one account, Hightower thought they had. "She was blindsided. As of a few weeks ago, everything seemed fine," an anonymous inside source told celebrity gossip site Radar Online. That inside source went on to say that Hightower, known as a socialite and philanthropist, had angered De Niro with her spendthrift ways. 

The famously press-shy couple has not commented on the reason for the split, or even publicly confirmed that they have split, although at least one source close to them says they've been living apart for some time, and De Niro appeared solo at the Friars Club roast for Billy Crystal. But whatever the true reason for the breakup, the important lesson for every couple who wants to stay together long-term is very clear: Do not ever let yourself think that your marriage or partnership is settled for good. It's never one less thing to worry about. You have to worry about it always.

I've seen it happen more than once, and so have you. When I was in high school, my boyfriend's parents had what seemed like an ideal relationship. They'd paired up while broke students in Paris, married six days after he proposed. They raised five kids and seven Siamese cats in a rambling apartment in New York's Upper West Side, back when it was affordable rather than tony. They had a small house in Connecticut that they had constructed themselves. They took fun trips to exotic places with the whole family in tow. Both had busy, fulfilling careers, she as a book illustrator, he as a sociologist. But then the youngest of the kids went off to college, and she announced that she was leaving, that she had been unhappy for years. From what I heard, her husband was blindsided too.

Another couple I know married when she was only 17 and he was 25, mainly because she wanted to get away from her father's home. They knew they might be too young for marriage, so the pair agreed that they were only committing to it for six months, at which point they would reconsider their options. After six months, they decided to go on for another six months, and then again, and then again, as the years piled up. They were still at it, and still happily married, when he died 64 years later.

I think they were on to something. If you want to make a partnership last, especially through the tough times, the overloaded schedules, the 2 am feedings, the preschool years that strain so many relationships, and everything else life throws at you, then you can never put it on the back burner. You have to choose your partner every single day, and he or she must choose you. If something's wrong, you have to find out what it is, and fix it, or talk it out. You might make a fuss over birthdays, anniversaries, and Valentine's Day, or you might not. But you do have to find the occasions for fun and romance, for gift-giving, and going on adventures. You have to find goals you both care about and work toward them together. You have to share each other's secrets, and triumphs, and disappointments, and it has to happen every day.

It takes all that, and more, to keep a relationship alive over time. It isn't easy to do, and it's way too easy to do the opposite. If you think your relationship is all set, you might be tempted to focus all your attention elsewhere, especially if you're an entrepreneur or an executive in our relentlessly busy times, when work can leak into every moment of our lives. But very bad things can happen to good relationships when either partner stops paying attention.

So it's a simple choice. Either you put in the work, or you risk winding up like my old boyfriend's father, and maybe like Grace Hightower, wondering how things went so wrong while your attention was focused elsewhere.