 This is a spiritual experience. It's something similar to what you would call the dark night of the ego. What happens is that back and forth between that person and yourself pushes you increasingly towards yourself. The moment you are in a state where there is no more pain, the person will come back. But if for some reason you move into a dependency relationship even a little bit chasing after that person, again they'll go. This is a specific type of relationship to teach you something specific. And that something specific is to be with source, with self, with the antar guru, with the master of the being in complete surrender.