It is based upon a simple, but profound, premise: that each relationship we have is either I:It or I:Thou. I:It relationships with things - I:Thou (I:You) relationships are those with beings. It is contained, inanimate, what you can experience. Thou is spirit, limitless. "If I face a human being as my Thou," Buber says, "and say the primary word I-Thou to him, he is not a thing among things, and does not consist of things."
This is what we do when we get married. We vow to always see the spirit in the other person, to understand and to relate to that person with your whole self.
And by the time couples get divorced they are relating to each other as Its - as things, as obstacles.
But when we treat someone else as an It we dehumanize ourselves.
In mediation, we ask parties to consider the whole person again, to treat that person as I:Thou, if only for the small time they are in the room. And it is by seeing that person's spirit, by treating them at their highest, that you honor your own spirit. For only the whole can relate to the whole.