Bay 12 Games Forum

In the beginning of the game, Toriel tells you that she goes through the ruins every day to see if anyone has fallen down, and comments that you are the first human to come for a long time. Now, one might be forgiven for assuming that she walks around looking for humans every day, and perhaps she does. On the other hand, there have only been six other humans since Chara showed up, and while we don't know how much time passes between each human entering the Underground it is likely several years. After all, almost no one you meet outside the ruins seems to be aware of who Toriel is, which implies that she's been there for so long that either everyone has forgotten she exists, or that whole monster generations (however long those are) have passed and few now living would know that Asgore was once married.

So anyway. Does Toriel really check the entrance to the Underground every single day on the off-chance that a human will show up? It's possible, but I have a more depressing theory. As you proceed on the pacifist route, you learn that when monsters are near-death (presumably from natural causes), they "fall down". Alphys used several fallen-down monsters in her determination experiments. It's likely that plenty more monsters fall down over time as well.

I submit to the jury that when Toriel takes her daily walks to look for people that have fallen down, she is not checking for humans. Rather, she searches for monsters that are going to die. For what purpose, I'm not sure, but given that she seems to be able to enter and leave the ruins at will (something most other monsters apparently can't do, if the Snowdin store owner is to be believed) my guess is that she brings the ashes of deceased monsters to friends or family or even random locations outside the ruins, should they wish it. She serves as a sort of benign Angel of Death, unable to save anyone (and frustrated by that) by able to provide comfort to the soon-to-be-deceased.