Dyadic Relationship Theory (often associated with Martin Buber’s I-Thou concept or various relational psychotherapy models).

In dyadic relationship theory, a relationship between two people creates a “third entity” - not just Person A and Person B, but also “the relationship itself” which has its own character, dynamics, and needs. This relationship entity influences how each person behaves and understands themselves.

I read about this in a psychology of close relationships class in college that forever changed how I relate to people.