Fear
Men are swayed more by fear than by reverence.
-Aristotle-
The face is so familiar to him, yet so foreign. The eyes, filled with sadness and anger. He nods slowly. Yes. Here is his deepest fear, his nightmare. The man on the other side of the doorway watches him. Hands that have brought the death of millions clench and unclench. The darkness to his light. The man he thought had died so many times. The Doctor reaches out, pulling the door closed. Darkness consumes the mirror once more.
My first drabble - never realised how consticting a hundreds words is!
Here I played on the idea that maybe what the Doctor fears the most about the Master, is that he reminds the Doctor of the darkness within himself.