When we're in conflict, we create a story in our mind of what happened to us. The details vary, but the basics are fairly consistent: we were minding our own business or doing our best when someone came along and messed everything up. We have tried to make it work with this other person, but he or she is completely unreasonable and unbending. We are the victim of someone else's craziness. When we're being completely honest, we might admit that we had a tiny part in all this, but ultimately, the other person bears the bulk of the blame.
We tell this story to our friends, family, and pretty much anyone who lends a sympathetic ear. Each time we tell it, we're met with a look of amazement, a "poor you" and a pat on the shoulder. This gratifies us because it confirms our belief that a) our story is real and unbelievably terrible and b) we were wronged. Telling our story and getting this response becomes addictive. We try to be careful but the story becomes central to our lives and sometimes we even tell it when our kids can overhear it.
As this story solidifies in our mind, it confirms our fear that we're powerless, it increases our sense the offending person is a monster and it builds a wall between us and them. None of this helps to create a strong or healthy co-parenting relationship.