What does Victors father say is a responsibility of the survivors?

From the text:

"Do you think, Victor," said he, "that I do not suffer also? No one could love a child more than I loved your brother"—tears came into his eyes as he spoke—"but is it not a duty to the survivors that we should refrain from augmenting their unhappiness by an appearance of immoderate grief? It is also a duty owed to yourself, for excessive sorrow prevents improvement or enjoyment, or even the discharge of daily usefulness, without which no man is fit for society."