This quote is pretty amazing and goes on to say:
“As for the the convenience or comfort of the single plate rule, no one can deny it who has ever looked upon an array of hungry guests whose eager appetite for the coming roast has been forced to an impatient delay by some social monster capable of asking twice for soup.”
You heard it here: Asking for second helpings of soup = social monster. (Rawwwwr!)
It's good etiquette to share what you like!