A while back, I received a sad letter from a reader. My correspondent was the wife of an entrepreneur who had been slogging away for six years at a start-up that she called a "money pit" and "marriage destroyer." She confessed that she hated her husband's business--more specifically that she hated what it was doing to him. "He becomes so criticizing and antagonistic when he is stressed, and lately it's been a lot," she wrote. "This is not the easygoing and very supportive man I married!"

I am no stranger to spousal laments. But what struck me about this note was the reader's assertion that entrepreneurship had actually changed her husband's character and temperament. I had taken it for granted that the person who goes into a business is also the person who comes out of it--maybe richer, maybe poorer, certainly battle scarred, but fundamentally the same. That was certainly true of my husband, Gary, who after three decades leading Stonyfield Farm is very much the energetic, idealistic, cynicism-free man I married.

Curious, I started reading about personality change. And I learned a few things, among them that the brain is not static. Rather, it is constantly rewired by our environments and by deep emotions. Entrepreneurship frequently evokes profound psychic responses: Elation and depression can succeed each other so rapidly as to induce emotional whiplash. In addition, entrepreneurs often suffer from intense and enduring anxiety. These emotions, and their dread companion, fear, can shrink human experience and any expansive sense of possibility. It takes extraordinary character and resilience to live with that every day and remain unaffected.

At least as powerful a force as fear is love. In his book 6 Secrets to Startup Success, business consultant John Bradberry contends that entrepreneurs literally fall in love with their companies and that such powerful emotion can alter personalities. If that sounds strange, try reading Much Ado About Nothing and Othello and mentally subbing in a start-up sports marketing agency for Beatrice and Desdemona. You begin to understand the transformative effects of company love, for good or for ill.

In some instances, entrepreneurship doesn't change people so much as reveal their true selves. Entrepreneurs learn by accumulating new knowledge and skills, Bradberry told me, but they also grow, by "stripping away accretions and misperceptions about who they really are." He explained that people who respond to adversity by getting in touch with their existing strengths create "a positive cycle of success and confidence and feelings of accomplishment and fulfillment." Unfortunately, some get in touch with their existing weaknesses. Beaten down by repeated disappointments, they are unable to hide the anger, petulance, despair, or other sour traits they had once suppressed.

Many people I spoke to for this column confessed that launching a company had, for whatever reason, made them less tolerant and more competitive, rigid, demanding, and critical. One woman told me that business building had turned her into an egotist and provoked unprecedented anxiety that led her to abuse alcohol for the first time. Another entrepreneur told me her entire outlook on life changed after a few employees cheated and lied to her. "I was once a trusting, loving, and giving individual," said Tammie Umbel, CEO of Shea Terra Organics, a natural skin care company based in Sterling, Virginia. "I see life more like a business now and carefully size up anyone I meet. I trust no one."

Not all entrepreneurs are sufficiently self-aware to notice the new face in the mirror. But the people who care about them cannot ignore the changes. Spouses whose mates became disagreeable, short tempered, hypercritical, or uncommunicative described family dynamics as a game of dodge ball, with everyone scurrying to avoid the entrepreneur's unpredictable temper. "My husband's moods are up and down depending on what happened in the business that day," one woman told me. "It affects the kids, too. My seventh grader is sensitive and doesn't want to rock the boat. I don't know if he'd be as quiet and withdrawn if my husband wasn't often like that, too."

A New Jersey woman told me that after her husband lost his business, he also lost his sense of humor, became disconnected emotionally, and withdrew from the family. The household has "no joy, no whimsy" in it, she said. She is close to asking for a divorce.

I would hazard that adverse personality effects are especially apparent in those who have long cherished the idea of entrepreneurship as salvation, the thing they were meant to do and finally would do that would lift them above the discontents of their normal lives. "My real life starts when my business starts," they assure themselves. The business starts. The business stumbles. What is left for them to dream about?

Fortunately, there is a sunny side to this particular street, at least for those lucky folks who thrive on pressure. The stress of daily life-and-death decision making is as likely to be confidence enhancing as debilitating. Starting a company makes some people more dynamic, focused, and creative. That, too, affects domestic relationships. No, this is not the man you married. This guy's better.

Such felicitous changes were especially striking for Gloria Sharrar, who after 36 years of marriage thought she knew everything about her husband. That's because she had been exposed only to the Dave Sharrar who worked for someone else. After a layoff, Gloria and Dave founded City­Park­ing, in Richmond, Virginia, and Dave began displaying levels of energy and motivation that were new to him. "I feel like I rediscovered Dave," ­Gloria told me. "I asked him, 'Were you always like this, and I just never noticed?'"†" The business has caused "a profound conversion" in their relationship, Gloria said. "It's increased my respect for him and made us a better couple."

Similarly, entrepreneurship revealed the warmer side of Larry Yatch. As a former Navy SEAL, Yatch had been trained to avoid personal interactions: to stand back, observe, and assess. Since he and his wife, Anne, started Sealed Mindset, a firearms and personal-safety training company in Minneapolis, Larry has become more fully engaged with those around him. "Being in business taught him about human relations," Anne said. "It's helped Larry develop as a human being."

These new-and-improved entrepreneurs aren't just easier to live with. They may also reinvigorate the romance in their marriages. "The change makes him so much more attractive to me," said Anne. Or as another spouse confided about her husband's newfound dynamism: "It's sexy as hell."

You can't tell in advance how starting a company will change someone's personality or whether it will have any effect at all. That is just one more unknown in the seemingly endless procession that comes with the territory. But though most founders fully appreciate the financial risks they are assuming, they routinely underestimate the risks to their emotional well-being and the health of their relationships. Just as they are obliged to detect and correct cash-flow or sales problems in their companies, they must also acknowledge and arrest their own slides into a psychic abyss.

One influential branch of entrepreneurial studies, dubbed effectuation, has coined the term affordable loss to describe what a founder is willing to risk and what he is willing to lose. The one thing he can never afford to lose is himself.