Kafka once wrote this to his fiancée:
"you once said that you would like to sit beside me while i write. Listen, in that case I could not write at all. For writing means revealing oneself to excess; that utmost of self-revelation and surrender, in which a human being, when involved with others, would feel he was losing himself, and from which, therefore, he will always shrink as long as he is in his right mind… That is why one can be alone enough when one writes, why there can never be enough silence around one when one writes, why even night is not enough."
in Location 1537 of the book Quiet: The Power Of Introverts In A World That Can't Stop Talking by Susan Cain