Board Thread:General Discussion/@comment-4504826-20160516034744/@comment-28554555-20160525230213

Nothing was ever said that ALL humans have enough determination within them to use SAVE files. They have greater determination than monsters ever could have without dying, but not enough to SAVE. I like to think that you have to have a certain threshold of determination to be able to use a SAVE file (when Flowey was created, he got just barely enough to SAVE). Frisk's determination to leave the Underground (and later to defeat Flowey, save his friends, and free everyone..or kill them) far surpassed this.

I think during Asriel's fight, Frisk and Asriel have the same level of determination. Asriel has the combined determination of seven human SOULs, while Frisk has far more than they originally did when the first entered the Underground...enough to overcome death itself. When Asriel says "Once I defeat you and gain complete control of the timeline", he means that neither of them are able to reset, load, etc. That's why you couldn't do the logical thing and, I don't know, reset the game, go through the True Pacifist but warn Papyrus not to trust Flowey, thus avoiding the entire Asriel situation (but that wouldn't give us our happy ending, would it?).