Italian researchers published a research last November that suggests the condition of "falling in love" appears to be associated with elevated levels of a nerve growth factor in the bloodstream.

Enzo Emanuele of the University of Pavia and colleagues suspected that a diversity of biological mechanisms might be involved in precipitating the mental state called "falling in love," a state characterized by obsessive thinking about one's beloved, craving for union with him or her, euphoria, and increased energy. And one of the mechanisms that the researchers thought might be implicated are neurotrophins—brain chemicals that nourish nerves and that have been increasingly recognized as potential mediators of anxiety and other emotions.

Using a Passionate Love Scale as a tool for measuring romantic love, they compared the level of the neurochemicals between 3 groups.

The researchers measured blood levels of four kinds of neurotrophins—nerve growth factor, brain-derived neurotrophic factor, neurotrophin 3, and neurotrophin 4—in 58 subjects who had recently fallen in love and two control groups. One control group consisted of 58 individuals who were not engaged in a romantic relationship; the other control group consisted of 58 persons who had been involved in a romantic relationship for 2.5 years to 5.5 years. The subjects in love were likewise assessed for anxiety and depression with the State-Trait Anxiety Inventory and the Beck Depression Inventory and for the intensity of their romantic feelings with the Passionate Love Scale, thought to be a reliable means of measuring romantic love.

A month later, in December 2005. Researchers in the US used fMRI to highlight areas activated in the brain in people who were deeply in love. They found:

Activation specific to the beloved occurred in the right ventral tegmental area and right caudate nucleus, dopamine-rich areas associated with mammalian reward and motivation. These and other results suggest that dopaminergic reward pathways contribute to the "general arousal" component of romantic love; romantic love is primarily a motivation system, rather than an emotion; this drive is distinct from the sex drive; romantic love changes across time; and romantic love shares biobehavioral similarities with mammalian attraction.

The dopamine reward pathways in the brain are related to the feeling of pleasure and motivation. They are usually activated with rewarding experiences like food, sex and certain drugs. The US team suggests that "falling in love" as a mating behaviour evolved to allow our species to save energy and make it easier to find a mate for reproduction.