Being in love is a powerful experience unlike anything else. It's an altered state in which people think and act very differently than usual. Some people never get to experience it, but many of us do at least once in a lifetime.

Those who have experienced it also know that the powerful rush doesn't last forever. And when those feelings end, the relationship often ends, too. Yet many couples manage to move on from that stage to keep their love affair going.

We used to turn to poets for insight on the mysteries of love, but now we ask doctors and researchers. Science offers two basic ways of understanding love affairs. One is to look for what many different people in different love relationships tend to have in common. The other is to look at how chemicals in the brain mix to make us feel various emotions related to sex and love.

But first things first. Just what is it that makes two people fall in love, hard and fast?

(How has your relationship changed over time ? Talk about it with others on WebMD's Health Café message board.)

Madly in Love

Beginning in 1965, a psychologist named Dorothy Tennov began to study the state of being in love as something different from other ways that people love each other. In 1979, she published a book summing up her research, in which she coined a new scientific term for "in love." She called it "limerence." Based upon hundreds of interviews with people in love, she came up with a general description of the condition.

• In the beginning, we become very interested in another person.
• If the other person seems interested in us, we become even more interested in that person.
• We feel a keen sense of longing for the other person's attention.
• We become interested in only that person and no one else.
• Our interest develops into an obsession: We can't stop thinking about the other person even if we try to concentrate on other things.
• We daydream and fantasize about the other person constantly.
• The relationship causes euphoria -- an intense "high" or feeling of joy and well-being.
• We think about engaging in sexual activities with the other person. 
• Sometimes we feel an aching sensation or pain in the chest.
• We fail to notice or refuse to acknowledge any faults in the other person, and no logical argument can change our positive view.

This Is Your Brain on Love

Researchers have looked for changes in the brain that may go along with the state of limerence. Studies show that the brain chemicals dopamine and serotonin may be related to the peculiar feelings and behavior of people in love.

Dopamine is a feel-good brain chemical. When the brain is flooded with dopamine, we feel various degrees of well-being, from contentment to euphoria. High dopamine levels may be related to the "high" people experience early in a love affair. People in love also tend to notice less need for sleep, extra energy, and decreased appetite. Some scientists think it's no coincidence that these are also common effects of amphetamines and cocaine, which alter the mind mainly by raising dopamine levels.

The downside of high dopamine is anxiety, restlessness, and emotional volatility. Such bad feelings are often mixed up with good ones in passionate love affairs. Dopamine plays a role in our ability to concentrate and control our thoughts, so elevated dopamine levels could explain lovers' tendency to focus exclusively on their beloved.

Because low serotonin in the brain is related to obsessive disorder, some scientists think low serotonin is a likely explanation for the way people in love obsess about their beloved.

Falling in love has been linked to hormonal changes, too. Researchers in Italy who studied serotonin and love affairs compared hormone levels of people recently fallen in love and those who were single or in a long-lasting relationship. They found that women who had recently fallen in love had higher testosterone levels than those who had not recently fallen in love, and men in love had lower testosterone than those who had not. Both men and women who had recently fallen in love also had higher levels of the stress hormone cortisol. When researchers tested these people again one to two years later, their hormone levels were no longer different.

The "in-love" stage of a love affair typically lasts six to 18 months, and occasionally as long as three years, says Denise Bartell, PhD, psychologist at the University of Wisconsin, Green Bay. But it does wane at some point. People get used to loving each other, maybe in the same way that people develop tolerance to the effects of mind-altering drugs.

Cuddly Hormones

Something keeps people together after the thrill wears off, however. "At a certain point there's a crossover from passion to intimacy," Bartell says, although, "that's not to say there's no passion in a relationship after that." People keep loving each other in a special way, and they keep having sex.

It seems likely that hormones are involved in intimacy, which psychologists also call attachment. Some research points to oxytocin and vasopressin, hormones thought to give us the "warm fuzzies." These hormones may also play a role in bonding between human mothers and babies. Studies of small rodents called prairie voles show oxytocin hastens attachment in mating voles and may even have the power to make non-monogamous voles act monogamously. But it isn't clear if what's known about voles applies to the love affairs of adult humans.

Breaking Up

If people were simple creatures, the hormonal process of romantic attachment would keep all love affairs going strong after passing through the "limerence" stage. People are not simple, and many couples who were perfectly, blissfully in love a year ago have split up and are seeing other people today.

Love affairs that start with falling in love may be set up to fail. Initially the lovers are in denial about any faults their beloved may have, and they're impervious to logic should anyone else suggest that the relationship may be a bad idea. After "limerence" wears off, certain things become painfully apparent.

So-called "fatal attraction" is another reason why love affairs end. In fatal attraction, a quality that one initially finds attractive in a lover is the same quality that sinks the relationship. For example, we may fall for a person's delightful sense of humor, but then come to see it as flakiness. Attractive qualities are usually two-sided. If a sexy and charming partner cheats, it's because he or she is charming and sexy to other people, too. A thrilling person may actually be dangerous. A doting, attentive lover may be overly possessive.

Researchers studying the love affairs of college students found that fatal attraction was involved in one-third of breakups. Extreme qualities were most likely to be "fatal." Lovers who were attracted to partners who were very different from them were also more likely to split up.

Sharing Keys, Swapping Rings

Most people in long-term relationships end up getting married, if the law allows it. But in the United States today, couples commonly live together for a while first. According to surveys conducted 1997 by the National Opinion Research Center at the University of Chicago, more than one-third of adults in their mid-20s to mid-30s who had been married lived with their spouse before marrying. About 40% in this age group had ever lived with a romantic partner while unmarried.

Nevertheless, such arrangements are usually short-lived, lasting on average one year before the couple breaks up or gets married. Looking at it another way, the CDC's National Center for Health Statistics estimates that 30% of unwed couples living together will get married after one year and 70% will after five years. The likelihood of breaking up instead of getting married is 30% after one year and 49% after five years.

The University of Chicago surveys also found that about half of unmarried people involved a love affair thought they would likely marry the one they were with.

For many people in the United States, being married is no more "forever" than being in love is. The CDC estimates that two out of five first marriages will end in divorce or separation after 15 years. U.S. Census numbers show that only about half of those married between the years 1970-1979 celebrated a 25th wedding anniversary.

Sex and Marriage: "Seven Year Itch?"

Couples probably have sex with each other less often the longer they are married. That's assumed because surveys find that married people report having sex less often the older they are. The University of Chicago's survey shows married adults under age 30 say they have sex an average of 109 times a year. The average number drops to 70 times per year for forty-somethings, 52 times a year for people in their 50s, and so on.

The survey also shows that married people younger than 30 are those most likely to have sex with someone other than their spouse. But there's no clear increase or decrease as people age, and by extension, the length of marriage.

The recurring question about a "seven year itch" is a funny case of fiction taking on a life of its own. The Seven Year Itch is the title of a 1955 movie starring Marilyn Monroe, which refers to a pretend chapter title in a made-up book by a fictional quack psychoanalyst who claims that men tend to have extramarital sexual affairs after seven years of marriage. Prior to the 1952 debut of the Broadway play upon which the movie was based, the "seven year itch" was just a folksy name for scabies. (Scabies is a very itchy condition caused by tiny mites living in a person's skin. It used to be  hard to cure, and it could last for years.)

In general, infidelity is not rampant in the United States. In any given year, only 3%-4% of married people say they've had sex with someone besides their spouse. About 16% say they have ever done so.

The Long Slide

Over time, married people tend to become less and less satisfied with their relationship -- not something you'd want to mention when toasting a bride and groom.

"On average, the newlywed period is a high point in the history of the relationship," Benjamin Karney, PhD, a psychologist at the University of California, Los Angeles, tells WebMD. "From there, it's hard to get better," he says.

For many years, common wisdom stated that happiness in marriage followed "U-shape" course, declining gradually into middle age and then gradually increasing into the golden years. This idea was flawed because it was based on studying groups of couples at a certain point in time, then plotting satisfaction with age. "The people who have been married the longest are a select group," Karney says. "They're the survivors."

When researchers looked at what happened with certain married couples over a long time period, satisfaction didn't follow a U-shaped course. In fact, it tended to fall from day one and never went up. The steepest drops were at the very beginning and in late life.

On the bright side, the decline stays within a narrow range near the top of the satisfaction scale. On a scale where one is least and twenty is most satisfied, couples tend to start at about 19 and end up at about 16.

Ties That Bind

So how does a love affair survive and thrive?

Having good communication and keeping relationship problems in perspective are the quick and easy answers, "but they're the small potatoes," Bartell says. "How we chose our partners is most important."

But not all long-term relationships are carefully calculated. Some couples commit. Others "get committed" by circumstance or inertia. That can keep relationships on the shelf past their best-by date. "People have to be aware when these things are happening," Bartell says. "It may seem inconsequential that you get a dog with your boyfriend, but it's really not."

Making a conscious commitment is important. Research shows that solidly committed couples are less vulnerable to relationship threats than more uncertain couples are. Threats may include partners' potentially "fatal" flaws, hurtful things they may say or do to one another, temptation from sexy other men or women, pressure from anyone who disapproves of the relationship, and all the sundry misfortunes that can befall people.

In other words, a love affair has more staying power when lovers aren't questioning whether the other is "the one."

Take Heart

The big limitation of science in the study of love affairs is that it can't predict what will happen to any particular love affair. Averages tell us what's normal, but they flatten out all the tragic and triumphant love stories that have been sung about for millennia. That's a good thing if you have high hopes for love. The next great love affair could be yours.