It is all about fragility

A portrait is a window into another person's inner world. You look at the face, seeing the exterior, and sometimes, you stop at the façade.

It depends on whether the person wishes to reveal their plush, soft, and delicate depths to you, or if they have simply forgotten to mask them in a moment of pure, unguarded joy.

I like Peter Lindbergh's statement about a portrait of a person, that it's not the physiognomy of that person, it's actually the emotions of two people who have come together to take a photograph.