One major problem is the stumble—and I use that image because the same energies that push a pair forward can also knock them over. Think of a toddler hurtling down a path. Crying “stop!” often enough to keep him from falling will also stop childhood itself. You don’t root for the kid to get hurt but you do root for him to take the risk. You wish you could be so brave.

“Why do pairs break up?” I asked Diana McLain Smith, a family therapist turned adviser to leadership teams, and she told me that to answer the question, we have to go back to how pairs begin in the first place. 

“Opposites do attract,” Smith told me, “and people see in someone different from them the potential to create something they can’t do on their own. So they enter the relationship celebrating the differences. At some point, it’s inevitable that the differences pose challenges that people don’t know how to deal with. They find the exchange more noxious than creative. So now they have to negotiate differences that are not as happy making.”

But by responding to the difficulties, the partners may exacerbate them. “Say you have somebody who is extremely spontaneous and action-oriented,” Smith said, “and she connects with someone who is more grounded and reflective. The wild one finds the relationship centering, and the even-keeled person appreciates the stimulation. Over time, though, the more even-keeled person may feel overwhelmed by the randomness and the spontaneous person may start to feel more constrained. But the person who is more spontaneous will tend to amplify her needs— turning up the volume; maybe throwing tantrums or sulking— and the person who is more even-keeled mutes her needs. Now they’re in a pattern, and it’s self-reinforcing. The thing each one of them does brings out more of the thing they don’t know how to deal with in the other.”

At their worst, these destructive dynamics aren’t even recognized as dynamics at all—or as anything mutual. In a negative spiral, both partners are likely to attribute the problems to the other. “It’s like the fundamental attribution error,” Smith told me. “In relationships, when times are bad, people say, ‘We have a fundamental problem. It’s you.’”

One of my favorite end-of-relationship studies is by the sociologist Diane Felmlee. She asked people what had initially attracted them to an ex and what repelled them at the end. For about 30 percent of people, both answers were really the same, just cast in a very different light. One subject found his or her partner —Felmlee didn’t identify gender—“ sweet and sensitive” at first but later “too nice.” One partner was wonderfully “strong-willed” and later obnoxiously “domineering.” A partner with a great “sense of humor” later “played too many jokes. ” The study subjects seem to have lost their tolerance for the qualities that attracted them in the first place.

In the heat of the moment, each person wants to blame the other. The irony is that this is what makes the relationship seem unbearable in the end, because people come to feel helpless, Smith told me. “They come to think that the only way out is to exit.” 

“You mean to say,” I asked, “that people don’t really want out of the relationship itself, they just want out of the dynamic?”

“That’s right,” she said. “They don’t want to go away from the person, necessarily. They want away from the feeling.”

So this is the first answer to how creative work between people gets interrupted: by forces that arise organically out of the terms on which the work began. The people we’re drawn to may unsettle us. They probably should unsettle us. But the differences that arouse us can later come to seem impossible. Graham Nash, for instance, found David Crosby’s defiance invigorating— Crosby was the unruly yang to Nash’s yin. Yet Crosby eventually pushed Nash to the breaking point when Crosby’s willful, erratic nature became entwined with drugs and alcohol. 

The nadir, Nash told me, was a 1979 studio session where Crosby showed up strung-out, slit-eyed, covered in sores. “His voice was rough, husky; our harmonies were strained,” Nash wrote in his memoir. “I couldn’t vibe him out. I couldn’t anticipate him anymore.”

The music had always saved them, Nash told me, and he nurtured hopes it would again, even as Crosby nodded out during breaks. One night, the band warmed up for a new Nash song called “Barrel of Pain.” “We had a great jam going,” Nash recalled, “it was rocking like mad.” As the studio amps vibrated with the sound, a crack pipe Crosby had laid on one of them slid off the edge, fell to the ground, and shattered. “I had tried everything with David,” Nash told me. “I tried doing drugs with him. I tried doing less than him. I tried doing more than him. I tried staying away from drugs to watch him.” 

But what happened next was too much even for Nash. Crosby stopped playing to try to put the pieces of the pipe together.   

“That was it for me,” Nash told me. “That was my darkest moment. When Crosby stopped the jam, I just went, ‘Holy fuck.’ That’s when I realized that the drugs were more important to Crosby than the music.”

“It was as clear as it could be,” Crosby added. 

“Even at the time?” I asked.

“Oh no, not for me,” Crosby answered. “I was in a haze. But a few days later, Graham called me up and he said, ‘Listen, this is too crazy . . .’” 

Notice that Nash characterized the break not as personal exasperation but as a rupture of their shared faith. “We always promised that the music would be sacred,” Nash said. A mutual deference to whatever is sacred allows a pair to abide. It keeps them upright.

“I’d say that the key word is humility,” South Park staff writer Vernon Chatman told me, reflecting on Matt Stone and Trey Parker, who, as I finished this book, had just wrapped their seventeenth season of the show, launched the fourth production of The Book of Mormon, and formed their own studio. “When two people come up against each other,” Chatman said, “it’s just natural that their egos are going to get battered. Humility is the thing that makes you recognize, ‘That’s my ego and has nothing to do with the larger issue.’ It’s just that thing where, ‘There is me, there is you, and there is us.’ And us is the biggest thing and you have to be reverent to it.”

Chatman paused. “It’s funny, because I’m saying all this shit like ‘humility’ and ‘ego’ and ‘reverent,’ but if you asked Matt and Trey they’d just say: ‘It’s like Van Halen. It’s like a band. And the band is bigger than anyone in it.’”

Nash called it faith, Chatman humility. This devotion to the partnership may also come down to loyalty. No matter how far Elizabeth Cady Stanton pushed her, Susan B. Anthony remained at her side. “Like husband and wife,” Stanton wrote, “each has the feeling that we must have no differences in public.”

Irritations and divergences between two people are as inevitable as the chattering of the mind in meditation; for a partnership, the equivalent of returning to the breath is returning to the joint purpose.

But this may grow more challenging over time, because annoyances can increase with exposure, and with success. J.R.R. Tolkien was uneasy with C. S. Lewis’s tendency to proselytize even when they were exchanging pages as young men in the 1930s. By 1947, after publishing fifteen books and running a radio broadcast during the war, Lewis had become, Time declared in a cover story, “one of the most influential spokesmen for Christianity in the English-speaking world . . . for his listeners, almost as synonymous with religion as the Archbishop of Canterbury.” This pop theology aggravated Tolkien, who believed preaching should be left to the clergy, and it contributed to a severe strain between the writers. 

Irritants may begin as generative— like the proverbial sand grain in the oyster— but, over time, become merely irritating. Dean Martin and Jerry Lewis’s act was built on Jerry looking to get a rise out of Dean, and Martin dressing him down or flicking him away. In one sketch, Martin (playing himself) is literally driven mad when every character he encounters— from the taxi driver to the man on the street to the psychiatrist himself— is Jerry Lewis. For years, there was palpable pleasure in Martin’s exasperation. But as time wore on, the exasperation took on a life of its own.

One reason relationships can tip quickly is that both positive and negative stories tend to be reinforcing. The psychologist Sandra Murray has found that, early in relationships, stories often skew to the positive. The presence of illusions—“ motivated misunderstandings,” Murray calls them— are positively associated with “greater satisfaction, love and trust, and less conflict and ambivalence in both dating and marital relationships.” And people not only tell themselves illusory stories, Murray found, but react to threats by deepening their attachment to them. That is, A not only idealizes B but reacts to troubling qualities in B by burnishing that ideal.

What often happens at the end of a relationship, Murray suggests, is that this feedback loop gets broken. Somehow, the troubling qualities of B are just too much for A to bear. And with the idealized narrative in tatters, a new narrative is constructed, casting the once-heroic, do-no-wrong partner as a goat who can do no right. Both stories, Murray argues, are ultimately meant to be protective— the first sort to protect A from worry and doubt, the second to protect A from the discomfort of ambivalence.

This helps explain the rapid fall of a partnership like Martin and Lewis, who went, in what seemed like a blink of an eye, from rambunctious friends to wary enemies. At their nadir, Lewis wrote in his memoir, he steeled himself to approach Dean and told him that he really thought what made their partnership magic was “the love that we had— that we still have— for each other . . . 

“He half-closed his eyes, gazing downward for what felt like a long time,” Lewis wrote.  “Then he looked me square in the face. ‘You can talk about love all you want,’ Dean said. ‘To me, you’re nothing but a fucking dollar sign.’”

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The above is an excerpt from the book "Powers of Two: Finding the Essence of Innovation in Creative Pairs" now available on Amazon.