Recently I was stunned to hear that a couple I have known – and, to tell the truth, envied – for years have gotten divorced. They always seemed so loving, so intimate, so supportive with each other. For over twenty years, they worked together, they traveled together, and I would often see them jogging together – in an uncanny synchronicity of strides. What went wrong? Like everyone who hears of such an unexpected break-up, I want to find out; and not only because of their marriage, but to understand grown-up marriage itself, my own included.

It is a truism that no one knows what goes on in another person’s marriage, but that doesn’t keep us from trying to figure out why some marriages fall apart and others stick. The “why does she stay married to him” question has come up again recently in the context of the Spitzer meltdown. Certainly the stricken face of the disgraced governor’s humiliated wife Silda will haunt many of us for years to come. Her blank mask as she stood behind him on his Day of Shame recalls the iconic photographs of other women in other circumstances, the widows of assassinated leaders – Jackie Kennedy, Coretta King, Ethel Kennedy – at their husbands’ funerals. Behind the mask, what was each of those women thinking? Was she thinking about what her marriage was really like? What her husband was really like in the privacy of their “understanding”?

We all know less extreme situations that still seem mysterious at the core. The smart, attractive, successful woman who stays with the philandering n’er-do-well or the upbeat, charming, generous husband who stays with his sour, narrow-minded, zenophobic wife. We assume that the one is putting up with the other, but given that we are exploring a mystery, we have to consider the possibility that the situation is reversed. Or there may be something that is very precious at the heart of their relationship – secrets shared, good sex, trust (even in the midst of betrayal?) – that makes all the rest secondary. When Hillary Clinton wrote in her autobiography about the dynamic of her relationship with Bill that they had “started a conversation” in 1971 and that it was still going on, she demystified that particular marriage to my satisfaction.

What makes a marriage endure? Do the partners themselves know what keeps them together? Can anyone say whether it is “good” for either or both of them? Is endurance a marital virtue? Or long shared history a reward? Such questions are particularly pertinent as we outgrow our youth. Children move on, for some of us our work moves into the background while for others it becomes foreground, and the place of her marriage in a woman’s life is open to new scrutiny. We know that two-thirds of over-age-fifty divorces are initiated by women. And most women can think of a hundred reasons why a break-up might happen then. But we know so little about the marriages that stick.

I suspect it has a lot to do with how the people involved are incorporating the changes taking place in their own lives – or how committed they both are to fending off change. I am sure that trust and judgment and laughter play a part, but there must be an ineffable something else – at least in the nurturing unions – that refreshes their curiosity about each other and about how things between them will continue to turn out. My guess is that when it comes down to it, we on the outside aren’t the only ones who don’t know what makes a given marriage work. Perhaps the intrigue of that mystery is the spark that keeps the partners engaged.