The third person in a marriage is the little nagging habits that you tolerate when it is trivial, but in the long run, becomes the straw that breaks the camel's back. Wu cited four major "third persons" in a relationship: nagging, tuning out, disinterest, and fastidiousness. Noted for her down-to-earth advice, Wu is a bonafide Dear Abby of relationships and has published over 20 bestselling books