The big and general answer (applies to spouses as well) is that the disagreeing parent (or disapproving spouse) can intuitively sense that this will change the systemic balance of the family / relationship. People are only together because they are a psychological fit, and once one member starts seeing the dynamics for what they are (and growing out of them), it upsets the apple cart. Other members must change or the relationship will end. And people are afraid (unconsciously) of change. Much of our nervous system is crafted by evolution to prevent it.
It’s called “[homeostasis preservation](https://www.quora.com/search?q=homeostasis%20preservation&author=84795924 “www.quora.com”)” and I’ve written a ton about it in these posts.
Another reasons one parent may object, is that they feel shame for their (perceived) oversized contribution to the child’s problems, and they fear they will be “outed” by the child to the therapist.
homeostasis preservation posts