Their relationship is so complicated because each of them has very different issues. I don’t have the patience to wade through them and type them but, in simple terms, Mira is manipulative and will do whatever she can to get what she wants. Jonathan is weaker but, through time and therapy, has learned to live with the pain of Mira ending their marriage and is now all about moving on. He knows that another turn with Mira is not in his best interest.
More importantly, Isaac’s and Chastain’s performances are amazing. They are Jonathan and Mira.