According to the New York Times, scientists have scanned the human brain and enpictured new love as it blossoms there -
"It is closer in its neural profile to drives like hunger, thirst or drug craving, the researchers assert, than to emotional states like excitement or affection. As a relationship deepens, the brain scans suggest, the neural activity associated with romantic love alters slightly, and in some cases primes areas deep in the primitive brain that are involved in long-term attachment."
http://www.nytimes.com/2005/05/31/health/psychology/31love.html?incamp=article_popular
How did Shakespeare know?
"Love is merely a madness, and, I tell you, deserves as well a dark house and a whip as madmen do: and the reason why they are not so punished and cured is, that the lunacy is so ordinary that the whippers are in love too."
Rosalind, As You Like It, Act III, Scene 2