The parentified child can be triangulated into the relationship between husband and wife, losing both parents in the process. In the majority of cases, it's the mother who parentifies either her daughter or son, making that child her confidant, her ally, and her emotional support system. She typically does this during a trying time in her life: a rough patch in her marriage, menopause, the loss of her parent, or a frustrating stage in her career.
When a mother parentifies her child, the dynamic in her marriage changes. Her husband feels less important and less needed. His wife leans on the child for comfort, support, and guidance and not him. Left out of the picture, he begins to pull back from both of them.
This is precisely what happened to me when I was a preteen. My mother became convinced that my dad was having an affair and used me as her personal therapist: detailing her worries and insecurities, criticizing my father, and asking for my advice. This dynamic continued for six long years until I went to college.
In that time, the relationship between my dad and me dissolved. It was obvious to him that I had become my mother's ally and was being fed a regular diet of derogatory things about him. I lost a paternal figure because my father simply gave up trying to be a dad to me. I lost a maternal figure because my mom abandoned that role and let me mother her.
When parentified youngsters grow up and understand what was done to them, it's too late. They're left without a mom, a dad or, most tragically, both. That's why it's easy for them to feel alone and resentful, longing for the carefree childhood that they missed.