What does necessity mean?

Chapter 3 page 17 second paragraph. Before Victor's departure for Ingolstadt.

She died calmly, and her countenance expressed affection even in death. I need not describe the feelings of those whose dearest ties are rent by that most irreparable evil, the void that presents itself to the soul, and the despair that is exhibited on the countenance. It is so long before the mind can persuade itself that she whom we saw every day and whose very existence appeared a part of our own can have departed forever—that the brightness of a beloved eye can have been extinguished and the sound of a voice so familiar and dear to the ear can be hushed, never more to be heard. These are the reflections of the first days; but when the lapse of time proves the reality of the evil, then the actual bitterness of grief commences. Yet from whom has not that rude hand rent away some dear connection? And why should I describe a sorrow which all have felt, and must feel? The time at length arrives when grief is rather an indulgence than a necessity; and the smile that plays upon the lips, although it may be deemed a sacrilege, is not banished. My mother was dead, but we had still duties which we ought to perform; we must continue our course with the rest and learn to think ourselves fortunate whilst one remains whom the spoiler has not seized.