"Frisk, you're made of glass." Her voice softens a touch, but Toriel is settled on her decision. "You're also a child. My child. My child who bases far too much of themselves on helping others."

She's never been oblivious on that. Frisk has always been in her eyes a go-getter, a child with a gentle heart and great intentions to help. Learning about what she does know of Frisk's powers, their deaths- it gives her a clear idea of why Frisk does this. Why Frisk thinks they can always jump in and solve apparently unfixable problems. They've always had a second chance, at least in the underground to fix a mistake. It also gives her an idea of what it has done to Frisk.

"You need to learn, you cannot always save everyone. Nor are you obligated to at every sign of danger or distress."