For extra sadness, try listening to "Remember Me" by The Birthday Massacre after reading this comic.
I've seen a few stories that explain Frisk's choice to repeatedly reset the timeline by saying they were curious, or bored, or just enjoyed their journey through the Underground so much that they wanted to relive it.
Some of these stories were really good, and I'm certainly not bashing their creators. But to me, the "resetting for pleasure" explanation seems to come from a very player-centric point of view, and it ignores a few key differences between the in-game protagonist and the player:
For the player, Undertale ends when you leave the Underground. The only way you can continue to interact with the story and characters is to reset to the beginning of the game and replay it.
But for Frisk, that isn't the case. After they leave the Underground, life goes on, they can continue interacting with their friends, and there are endless new events and experiences to discover.
If they had to choose between Undertale's happy ending and reliving their journey through the Underground, the former seems like the less boring and more enjoyable choice.
The whole Undertale experience is probably a lot more fun for the player, who is sitting comfortably in their home, than it is for the protagonist, who gets stabbed, burned, beaten, and otherwise hit with various magic attacks on a regular basis, depending on the player's skill level.
Depending on their backstory, their trip through the Underground might very well have been the most brutal and traumatic experience of Frisk's life.
I doubt they'd repeat that experience for fun, unless they like pain, and it would take a powerful and twisted sense of curiosity for them to choose to endure that just to see what happens when their loved ones die.
And speaking of their loved ones...
When Frisk resets, they possibly lose EVERYONE.
While player-created backstories vary, there are signs - such as the bandage that's been used multiple times, the fact that you can flirt with Toriel AND call her "mom", and the fact that you can choose to go live with Toriel at the end of the game - that suggest that Frisk's home of origin may have been neglectful, abusive, both, or simply nonexistent.
If they don't have friends or family on the surface, then the monster friends they meet in Undertale are all they have.
Imagine a world where all your friends have forgotten about you. You remember loving them and being loved, and you still care about them, but to them, you're just another stranger. To some of them, you're even an enemy to be destroyed.
Sure, you can repeat the experiences that created your relationships. You can go through the motions. But interactions and experiences that came from your authentic emotional reactions will never be truly recreated, because this time you'll be faking it, and observant people will pick up on that.
You aren't completely the same person anymore, and the people who remember the events that made you who you are are gone forever.
All in all, resetting the timeline would probably be a miserable experience. It certainly is for Just Cause's version of Frisk. To them, time travel is a blade without a hilt: useful as a last resort, but it will inevitably hurt them.
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