Opposites attract, but when there are too many differences, opposites repel.  He likes to sleep with the windows open; she likes them closed.  He's a night person; she's a day person.  He prefers country western music; she likes classical.  She's well organized; he's a "take-it-as-it-comes" person.  Before they married, they both admired in the other the very characteristics which later became annoying.