Pick a second -- Gallifreya
Sans had let a fine layer of snow settle on him again. It didn't really matter. Skeletons were generally immune to the cold in the first place, and he just didn't care for the second.
Every time he opened his eyes, he saw the sky... Blue sky. Real stars. Clouds... it didn't matter.
Crowding didn't matter. Not any more. There were seemingly boundless areas of mountainside and forest in which to build homes, grow farms, and otherwise just be. Sans preferred New Snowdin. Everyone knew him, and he could rely on two or more residents to drag him and his lawn chair under the eaves when the weather got harsh.
It was worth it to open his eyes and see...
...the gem-speckled ceiling of the cold caverns.
Sans spent his first reset on the border of madness. Trying to find someone, anyone, who also knew about the reality that was.
Anyone except the kid. Sometimes, they were... terrible. Other times, they were an angel. And those times, he got to spend time above, gathering snow, letting rain pool in his eyesockets. Enjoying the moments before...
...he found himself staring upwards at a bleak imitation of the sky. Heartbroken.
He'd tried so hard, so often, to set things right.
In the end, he only cared when the kid had murdered everyone they could find. And even then, he didn't stay dead for long.
And sometimes, once in a great long while, the kid would speak the words that meant that they had the power to see alternate lives. He hoped they would somehow be able to fix the machine his father made. Set things right.
But what did he expect? They were a kid. All they knew how to do was torture him with the promise of the outside on a permanent basis.
And in the end... he just gave up. Kept pulling the same jokes. Kept working with the same script. Kept enjoying the time above while it lasted. Kept... not caring.
It hurt too much to care.
Even when the kid crawled into his lap and promised there would be no more resets, he kept waking up from the nightmare of seeing that gem-studded imitation of a sky.
Day after day.