Dr John Marsden, head of the National Addiction Centre at the Maudsley Hospital in London, said that when you are attracted to someone your brain releases the drug dopamine, giving the same reaction that taking cocaine or speed would create.

When we are attracted to someone part of the brain which processes emotions is fired up causing the heart to pound three times faster than normal and cause blood to be diverted to the cheeks and sexual organs, creating the feeling of butterflies in the stomach.