Could be because they blame their own negative emotional reactions to the circumstances around their relationship on the other partner. This becomes so habitual, and so un-examined, that they actually believe that the other person is causing their unhappiness rather than their own thinking. It is a case of our ego-mind needing stimulation and drama and it (the ego) believing that without that stimulation it will die.
The challenge is, as usual, "Know thyself." Unfortunately, nearly everyone in the world still believes they are their thoughts and their attendant emotions. Until that fundamental mistake is realized the drama in relationships will continue.