1 reset? That was fine with him, maybe Frisk just made a mistake and wanted to change it.
2 resets? Frisk gave them all a happy ending. He trusted them for that. Everyone always thanked them.
3 resets. That's when things started to go downhill. They must've been curious, about what would happen if they killed everyone in the Underground. Sans had tried to stop them, but their Determination was too powerful.
The 4th time, something changed about Frisk No one could see it but Sans. He just didn't know what was wrong yet. Once again, they killed everyone, including him.
It wasn't until the 6th time they reset and did genocide that he figured it out.
They aren't doing this because they're curious... They're doing it just because they can.
And because they can, they have to.
Frisk kept resetting, and every time it was nothing but genocide. Every time Sans had to watch his friends and his brother die.
But then they noticed...
365 resets later, Sans was the one who changed. And not for the better.
Once again, he had been beaten. Frisk expected the same line that meant nothing to them anymore. At first, it was heartbreaking, and they wanted to RESET. But then Chara took over, and everything changed.
"So, kid. I know why you're still doing this. You're just too Determined, aren't you? Well, don't expect next time to be so easy."
They were confused, mostly. They thought nothing of it.
"Only one more time until we have one RESET for every day of the year."
And so, Frisk RESET. For the 365th time.
And everything had changed.