Can someone explain what Schopenhauer means here?

He is talking about why we can understand someones morals better without them speaking.

…secondly, that what is gained from an objective point of view by the clearer development of a man’s countenance while he is speaking, is again from a subjective point of view lost, because of the personal relation into which he immediately enters with us, occasioning a slight fascination, does not leave us unprejudiced observers, as has already been explained. Therefore, from this last standpoint it might be more correct to say: “Do not speak in order that I may see you.”

I’ve been reading this over and over again and can’t fully understand it. Please help me why he thinks this!