Frisk giving up their own SOUL to give Asriel a second chance is heartrending enough, but particularly the reveal that Frisk had reset the world countless times after their journey in trying to find a way to save Asriel, until sacrificing their SOUL became the only option available.
This exchange:
Asriel: Ha...Frisk...q-quit kidding around. You know I can't accept that. I-if I took your soul...th...then you'd...
Asriel, shaking violently, returns the hug in tears.
Asriel: ...You idiot...
The boss fight between Asriel and Chara ends with Chara using all the Frisk SOULS they had collected from other timelines to whittle Asriel's HP to 1 as Chara holds their knife for the killing blow...
Chara: I...I... (Crying) I can't...(Drops the knife)