They have realized that they really hardly matter in the big picture, there was never even a big picture.
Sure, if they are lucky they might matter to a few people, people who might depend on them or people who have worried about them, maybe for a long time, but perhaps those people have started to notice their worry has now turned into regret.
Realizing that they have seen enough of Spring turning into Winter and back again, enough of the Beautiful and the ugly and the continuing cycle of consuming and wanting and scrubbing the world clean.
Enough of trying, enough of not trying. Relapse, rinse, repeat.
What then? It's a matter of logistics, leaving the game, the stage, the good fight.
A final decision on what is the best way to leave a beautiful garden of memory or scorched earth in your wake on the way out, but the realization that you will be forgotten in time, and that the few who mourn your absence will be also forgotten in time, makes that decision even a more personal one.
Out there right now in the great big world, there is someone who has realized something important.
That one candle that burned in the darkness going out will not be missed by the light of day.