Perhaps it was because their first meeting involved Kate putting her career aboard the Enterprise in jeopardy in order to support Deanna in her unexpected pregnancy. Perhaps it was the sense of humor, poking harmless fun at others, especially the captain, whom Deanna had always thought could use some loosening up.
Or it could have been neither of those things, but simply the fact that they clicked.
Whatever it was, when they sat in Ten Forward, fingers entwined together, both found themselves basking in a contentedness neither had known before.