The answer lies not in the heart, but in the brain.

When you find someone attractive, the brain is flooded with dopamine and noradrenaline, which make you feel elated and alert.

Research shows that this can happen within a fraction of a second.

These chemicals appear to shut down some areas of the brain, like the prefrontal cortex, which regulates self-awareness and rational behavior.

The rush of dopamine activates the reward circuitry of the brain, the same areas that are active when humans feel a rush from cocaine.

In fact, the behavior associated with passionate love has a lot in common with addiction.

At the same time, the brain experiences a drop in serotonin, a chemical that regulates mood, which scientists believe causes behavior resembling that seen in people with anxiety and obsessive-compulsive disorder.

The intense feeling of passion at the start of a relationship often gives way to a more calm attachment.

The hypothalamus releases oxytocin and vasopressin into the reward system of the brain, which spark feelings of bonding and arousal.

One theory is that the intensity of love is, in fact, the brain tricking couples into bonding for the difficult task of raising children. What's more, one study suggests that the brain treats love as essential to human survival.

When participants looked at an image of their romantic partner, the part of the brain associated with fulfilling basic needs, like eating or drinking, lit up, suggesting that love, much like hunger and thirst, is vital to the continuation of our species.