Take a poll on what people believe is the most common reason for an extramarital affair and the answer you are likely to get from the majority is: sex.

While this may be a common belief, it's not necessarily true, says Andrea Syrtash, a dating and relationships expert. "For men it sometimes is," she says. "But most of the reasons that women cheat are not sexually driven. It's more about wanting to find attention."

Women want to be loved and appreciated, she says.  Men, on the other hand, are looking for approval. They may be trying to escape from a "nagging wife," as Syrtash puts it, who never thinks that what they do is good enough.  They want someone new who will think they are wonderful.

Furthermore, there's the lure of the excitement that comes with a new relationship, and that can be a major factor when the sparks in a relationship die down and don't come to life again.

Once this happens, boredom sets in and can contribute to the desire for an affair, says Dr. Michael Silverman, assistant professor of psychiatry at Mount Sinai School of Medicine in New York City, who estimates that half the patients in his practice either are involved in some sort of extramarital affair or are considering one.

"Early on, there is a lot of excitement as the relationship is forming, and we all love the crush," he says. "After being with someone for an extended period of time, that excitement diminishes."

Depression, too, can result in an affair, and women are twice as likely to be depressed as men. The post partum period is a common time for depression to strike: women are seven times more likely to experience a mood disorder after having a child, Silverman says.  When a person is depressed, he or she tends to withdraw and lose interest in sex. This can be the impetus the partner needs to seek solace and approval outside the marriage.

What You Can Do to Keep Your Partner by Your Side

Keep the lines of communication open, Silverman says. "A big reason why couples have problems is that they're not talking to each other," he says. "When you communicate with each other your feelings don't get to that point."

Seek out activities you both enjoy and do them often, Silverman recommends. And don't just limit these activities to an occasional weekend or vacation together (though these can be good, too.) Try to find experiences that you can learn and laugh about together. "It sounds hokey but it's when you have these shared experiences that you can bounce things off each other and talk about," Silverman says.

Renew your commitment to each other regularly, says Syrtash. "There will always be temptations and to think that there won't be is a little naïve," Syrtash says. "But you need to make the decision to stay committed, and work on this."

Don't place the blame on other people. "It just happened" or "My wife wasn't making me feel good" are lame excuses.

Carefully consider the repercussions before you get involved with someone, Syrtash says. Just thinking about how many lives you may be screwing up may be a deterrent.

If you are depressed, seek help. And if you suspect that your partner is depressed, encourage him to seek help, too.