People seem friendlier. Colors are brighter. Food tastes better. The world is transformed with shimmering newness.     What causes the rush of good feeling we call romantic love?     Scientists have learned that lovers are literally high on drugs. Natural hormones and chemicals flood their bodies with a sense of well-being, as the brain releases neurotransmitters dopamine, norepinephrine and serotonin as well as endorphins and enkephalins. These produce a rosy...