For simplicity's sake, I refer to heterosexual women in this post, but what I discuss here definitely applies to heterosexual men and non-heterosexual individuals as well.

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Many of us are familiar with this scenario: Mr. Nice Guy is cute, sweet, interesting, smart, and available. Even better, he is interested in a relationship with you. The only problem is that you just aren't that into him. Mr. Bad Guy, on the other hand, is on your mind 24/7.

Like Mr. Nice Guy, Mr. Bad Guy has a lot of good qualities, but he is either unavailable for a relationship in general, or unavailable for a relationship with you, because he just isn't that into you. Despite his continual rejection, however, you cannot seem to get him off your mind. The more he rejects you and the more forcefully he indicates that he doesn't want to be with you, the more interested you seem to become.

Why do we develop this bad habit of wanting what we cannot have? Why don't we always want what we can have? In other areas of life, it seems that we can adjust our preferences to fit the situation. You may have once flirted with the idea of becoming a Hollywood star. But when you discovered you couldn't act, you let go of that dream (I hope). So why can't we let go of people who continually reject us?

According to Helen Fisher and her colleagues, the reason romantic rejection gets us hooked is that this sort of rejection stimulates parts of the brain associated with , reward, , and cravings. Using functional MRI, her team looked at the brains of 15 college-aged men and women who had recently been rejected by their partners but claimed to still be intensely "in love." During the scan, the research subjects looked at a photo of the person who had rejected them. They then completed a math exercise, such as counting backwards from 4,529 by 7. The exercise was an attempt to distract participants from their romantic thoughts. Finally, they were shown a picture of a familiar person they were not interested in romantically.

The team found that participants' brains were more active in areas associated with motivation, reward, craving, addiction, physical pain, and distress when they looked at the photo of the person who had rejected them than when they looked at the photo of the neutral person.

The study, published in the Journal of Neurophysiology in 2010, shows that people in this situation are really suffering from a drug addiction, and the drug is the person rejecting us, leaving our love unreciprocated. But the results do not give us insight into why we respond to romantic rejection in this way, and it doesn't answer the question of how we have developed this troubling tendency of wanting people we can't have.

You might think it is a matter of heartbreak and . But that cannot be the full answer either, because in some cases we haven't lost anything that we can grieve the loss of. We can be madly in love with someone who doesn't want us, and never wanted us but the situation can sometimes be as painful as someone breaking up with us.

In a previous post, I argued that part of the rejection pain we feel when love is unreciprocated may be caused by an evolutionarily-grounded repulsion to social rejection combined with a social stigma associated with breakups and . But that, too, does not explain why we often want only those individuals we cannot have.

Another aspect of this anguish may have to do with the perceived value of the other person. If the other person doesn't want us or is not available for a relationship, their perceived value goes up. They become so "expensive" that we cannot "afford" them. Evolutionarily speaking, it would have been an advantage to mate with the most valuable mate. So it makes sense that we become more romantically interested when a person's perceived value increases.

Another answer may have to do with our relatively addictive personalities. Fisher's study showed that anguish and pain after romantic rejection is a kind of addiction. The question remains, however, what is it we are addicted to in this scenario?

In the case of a relationship that has ended, we may be addicted to the time we spent with the other person, their text messages, their company, or the . But if our brains work similarly when our love is unreciprocated, and there never was a relationship, what is the source of the addictive feelings? Presumably, we're addicted to thoughts of what could have been but never will be. Once we get stuck on those thoughts, being rejected by the other person can intensify them, leaving us to deal with obsession, which is a kind of addiction—or an addiction to thoughts of a certain kind. Elsewhere, I have argued that standard methods for dealing with can also help you get over romantic obsession.

Your style can also influence how much you get stuck on people who don't want you. People with a dependent attachment style (also known as a or attachment style) are brought up to seek out people who will cause them pain. In a classic scenario, they grew up in a household with a mother or father who emotionally rejected them. For these individuals, being romantically rejected is a familiar feeling. Since we are always more likely to act in ways that are familiar to us, if we have a history of rejection, we are likely to seek situations where we should expect more rejection. Our brains interpret these scenarios as normal, even though we know that it is not normal to seek out scenarios that lead to pain and anguish.

Source: Oxford University Press, used with permission.

Finally, there is the "different ending" explanation: If we have a history of being rejected—by a parent, for example—we sometimes subconsciously seek out similar scenarios, hoping that the story will have a different ending next time. Only it does not. It is worth remembering Einstein's definition of insanity—doing the same thing over and over and expecting a different result.

Berit "Brit" Brogaard is the author of On