Why do Couples always fight?

And why do they pretend they don't around other people? You can sit in a room with a couple and watch them laugh and smile, but you know there is a mental score card going back and forth, and the the minute we... I mean "they" get out of the public arena you know it's all built up into the perfect drive home full of lovely insults, and cute threats of breaking up. When... you know if they wern't fighting they really wouldn't have anything to talk about.