Because we see people and things not as they are but as we are. That is why when two people look at something or look at someone you get two different reactions.

You cannot see what I see because you see what you see. You cannot know what I know because you know what you know. What I see and what I know cannot be added to what you see and what you know because they are not of the same kind. Neither can it replace what you see and what you know, because that would be to replace you yourself.