"Love at first sight" is defined as a condition in which someone feels romantic love for a stranger immediately upon meeting them. Brain imaging shows that when this happens areas of the brain involved with dopamine, a hormone and neurotransmitter, are activated. Dopamine has important roles in behaviour and information processing, in activity, motivation and reward, sleep, mood, attention and learning. Hence intense romantic love is associated with neuronal reward and motivation pathways in the brain, particularly in areas involved with addictive behaviour such as compulsive gambling.
Love deactivates a set of regions in the brain associated with negative emotions, social judgement and "mentalising" (assessing other people's intentions and emotions), and bonds individuals through the involvement of the reward circuitry.
Romantic love has much in common with behaviours in other mammals and in birds: it is the third of three behavioural repertoires associated with reproduction: sex drive (to find a mate), attraction (to find the best mate) and attachment (to allow time for successful reproduction). To science, romantic love is not a particular emotion and does not use a functionally specialised area of the brain. It enables the individual to make a more efficient use of time and resources for successful reproduction. It is believed that "love at first sight" is an extreme extension of the normal romantic bonding process.