When making a portrait of someone, you see yourself in that portrait, like looking into a mirror. I read something like that from an interview on American SuburbX (of Richard Avedon, maybe).
It's trust or doubt in the subject's eyes.
It's a cold or warm feeling toward the photographer.
It's not only the subject but your own interpretation of him/her.
I've just created a blog, Faces @ http://you-me-n-myself.tumblr.com/. Not really a "portfolio", just a place I come to see the face of someone again, or just to see myself..