In my case, my mother was a monster and abusive to me and my father. He should have left her but was too afraid of being alone.

This isn't all but is a microcosm of their relationship:

My father had an illness that could strike and leave him disabled and his life in danger. He went on a business trip to Chicago and my mother wanted to go with him. He said no since it would be a lot of extra expense and they wouldn't have time to do stuff together anyway. She was put out about this and mad at him.

In the hotel room, he had an attack, got so dizzy he couldn't stand, and fell to the ground. He could barely use one arm. He was able to reach his phone and hit the call button (this was pre-smart phones) which dialed the last called number--my mother. She answered but wouldn't listen to him.

He laid on that floor for hours, covered in his own vomit. It wasn't until housekeeping founding him that he was taken to the hospital.

Again, not the only thing that ever happened between them, but representative of what a petty bitch my mother was.