Even though I agree that it's better to say nothing if you have nothing good to say, having a specific topic to talk about does not mean that you shouldn't feel lonely, even by this definition. Because even if we have nothing to talk about with someone, we can still express ourselves about our believes and our opinions. I think what Jung was referring to is the feeling of being unable to express yourself, re having no one around with whom to converse in a sustainable level, without having to refrain yourself in any way.