Scientists say they have discovered what happens in the brain when someone falls in love.
The study looked at the brains of people in love
They studied chemical reactions in men and women who were all in the early stages of relationships.
Research, published by the Society for Neuroscience, found activity in areas of the brain which are linked to energy and elation.
But scans found women's brains showed emotional responses, while men's showed activity linked to sexual arousal.
Researchers took functional magnetic resonance imaging (fMRI) scans of the brains of 17 young men and women to see what was happening in the brain of someone in love.
They were alternately shown a photo of someone they loved and one of someone they knew, but were emotionally neutral towards.
In between, they were given a task to distract them from their emotional responses to the photos.
They found that feelings of intensive romantic love were linked to activity in the right caudate nucleus and right ventral tegmental areadopamine, which have high levels of dopamine activity.
Dopamine is a brain chemical which produces feelings of satisfaction and pleasure.
Elevated levels are linked to increased energy, motivation to win a reward and feeling elated.
The researchers also found activity in other areas of the brain changed - including one that another study showed was active when people ate chocolate.
The more romantic someone said they were feeling, the more activity there was in these regions.
But there were differences between the genders.
Most of the women showed more activity in the body of the caudate, the septum and the posterior parietal cortex, which are areas linked to reward, emotion and attention.
Most of the men in this study showed more activity in visual processing areas, including one associated with sexual arousal.
Dr Helen Fisher, of Rutgers University, New Brunswick, New Jersey, who led the research, said: "We believe romantic love is a developed form of one of three primary brain networks that evolved to direct mammalian reproduction.
"The sex drive evolved to motivate individuals to seek sex with any appropriate partner.
"Attraction, the mammalian precursor of romantic love, evolved to enable individuals to pursue preferred mating partners, thereby conserving courtship time and energy.
"The brain circuitry for male-female attachment evolved to enable individuals to remain with a mate long enough to complete species-specific parenting duties."
The researchers plan to carry out another study where they will take brain scans of men and women who have recently been rejected by partners, interpreting and responding to social signals.