Fisher and other scientists who study "philematology," the science of kissing, believe that the act evolved as a "mechanism for mate choice and selection." It stimulates sex drive, romantic love and attachment, the latter two helping us tolerate people even when we're not having sex with them, she says. Saliva is a powerful way to telegraph your chemical goods but be warned that extra drool could backfire if your prospective mate finds you biochemically incompatible. Hence, the term "kiss of death," says Fischer. Good luck.