Can science tell us why we fall, and stay, in love? For centuries, love has been celebrated - and probed - mostly by poets, artists, and balladeers. But now, its mysteries are also yielding to the tools of science, including modern brain scanning machines. At a university in Stony Brook, N.Y., a handful of young people who had just fallen madly in love volunteered to have their brains scanned to see what areas were active when they looked at a picture of their sweetheart. The brain areas that "lit up" were precisely those known to be rich in a powerful "feel good" chemical, dopamine -- the substance that brain cells release in response to cocaine and nicotine. Dopamine is the key chemical in the brain's "reward system," a network of cells associated with pleasure -- and addiction.