Doesn’t individual unhappiness mean that there has been a breakdown in the marriage?
Implicit in the idea of a breakdown in the marriage is the idea or notion that a failed marriage is an unhappy one from which the spouses should have opportunity to escape.
The idea of a breakdown in the marriage is a relatively recent development and was not considered adequate grounds for divorce until comparatively recently. In a roundabout way, the idea of a gets back to societal pressures that made for liberalization of divorce laws. The phrase breakdown in the marriage is big enough to include a situation where two spouses -- one is just unhappy with the status quo and wants out. Put another way, until very recently in America’s history, personal unhappiness was not a ground for divorce, and middle-class America reinforced the law with societal disapproval of divorce.