Why do fools fall in love? And when we do fall, why do our faculties of reason--and decency and self-respect and even right and wrong--sometimes not come along? For that matter, why would anyone reciprocate the love of a partner who has come so romantically unhinged? The thought of a loved one can turn our wits upside down, ratchet up our heart rate, impel us to slay dragons and write corny songs. We may become morose, obsessive, even violent. Lovesickness has been blamed on the moon, on t...