Think of the last wedding you attended. Did the couple’s vows to each other involve promises to be, for the rest of their lives, friends and family and companions and lovers and allies? Did the two people vow to keep exciting each other and soothing each other and listening to each other and challenging each other, to be co-adventurers and co-Netflixers and co-owners of things and possibly co-parents of things and, all in all, pretty much all things to each other?

If so, the couple is very modern. Marriage has spent most of its existence, in the West and elsewhere, as primarily an economic arrangement; as a result, it has also spent most of its existence much less laden with the emotional expectation that we tend to heap upon it today. The current romantic conception of marriage—evident in the culture not just in the guise of hopeful wedding vows, but also in the wedding industrial complex and in pretty much every Hollywood rom-com that defines its “happy ending” according to the satisfactions of long-term commitment—is the result of historical coincidence. It arose, the Belgian relationship therapist Esther Perel argues, from a collision of several forces that collided in the 19th and 20th centuries: among them capitalism, latent Romanticism, and the political and cultural notion of the primacy of the individual.

As a result, as Perel put it during a conversation at the Aspen Ideas Festival, co-sponsored by the Aspen Institute and The Atlantic:

For the first time in history, we want one relationship to give us all the needs that have to do with anchoring and rooting and a sense of belonging and continuity and stability and predictability and security and safety and that whole dimension of our life—and we still want that same person to also provide us a sense of novelty…. I want the same person to be familiar and to be new, and to be comfortable and to be edgy, and to be predictable and surprising.

What does all that amount to? For one thing, Perel argues, cheating. (Perel defines infidelity broadly: It could be sex with another person, but it could also be the more nebulous stuff of “emotional affairs.”) Though we often think of affairs in terms of deficiency—what went wrong, what one person couldn’t give the other, how one person failed the other, and all that—often, Perel has found in her clinical work and in her research, people cheat not because they feel confined in their marriage, but because they simply want more of what it proposes to offer. People evolve; marriage assumes that they will evolve together. That is not always the case. Cheating can be the it’s not you, it’s me result of that. Often, Perel explains, “it’s not so much that you want to leave the person that you’re with; it’s that you want to leave the person that you’ve become.”

That’s not necessarily to excuse cheating, which is a betrayal that causes pain that can reverberate over the long term—and not just between the couple in question, but also to their children, their friends, their community. (Perel asked the crowd in Aspen to raise their hands if they had been affected, somehow, by infidelity; a clear majority of hands shot up in response.) Instead, Perel’s aim is to explain why cheating is so common—why, indeed, it still serves as marriage’s darker corollary.

Part of all that, she believes, can be accounted for in those historically inscribed expectations of what marriage and long-term coupling offer—expectations that, more often than not, cannot be fully met by changeable, fallible humans. Cultural notions of what romance is all about fill us all with heady hopes; cheating is an attempt to fulfill those hopes. That, too, is new. It used to be, Perel said, that people outsourced their expectations of happiness to cultural institutions, organized religion chief among them. People found fullness in their lives—all the stuff of the modern-day wedding vow—not just from their spouses, but from community and civic engagement and religious faith. (Or, as Perel put it: “‘Happy’ used to be for the afterlife.”)

But American culture is increasingly secular, and more to the point increasingly self-directed, in a Bowling Alone and Culture of Narcissism kind of way: We will take our happiness here and now, thank you, and we will do whatever we can to get it. Cheating often stems from the psychological manifestation of that cultural attitude: It’s a logic of “I deserve this,” Perel explains. “It would be a betrayal to myself if I didn’t pursue that.”

Perhaps, given all that, it’s time for us to rethink infidelity—and, more to the point, marriage itself. Cheating, Perel said, “has a tenacity that marriage can only envy”; that alone might be a sign that something is amiss. Long-term monogamy can offer all the wonderful things that wedding vow-ers say it does; it is also, however, a social, economic, and political structure, one that has been less defined by human nature than by cultural expectations. And those expectations are, even in this age of delayed marriage and marriage equality, heavier than they have ever been before. Marriage, so inflated with hopes that used to be outsourced to other institutions, involves a series of bubbles; it is probably inevitable that some of them, at least, will burst.