I probably didn't. I couldn't. [He's a coward, for one, but he knows the reasoning, because his mind has traveled the well-worn lines of justification long enough to leave a mark on his soul, he's sure.] Nobody except the person time travelling can do anything about them. If I went and told you--or Toriel, or Alphys, or anyone-- [The emotion creeps back into his voice, squeezing it and breaking it up.] Knowing just makes you miserable. I've got a visualization at home, and I still can't count how many timelines there are. So many of 'em just stop. [Sans is crying, but for him it's more like tears are just happening. His voice is shaky.] Everybody's got all these hopes and dreams. If someone... Knows. That everything can just be gone. And you end up back Underground, back where you were, and you won't even know. It makes it pretty hard to put in the effort for anything that might happen in the future.

[A future that might never come. But now time has kept coming, and they're on the surface, at home, and Sans likes Frisk, and this place just keeps reaching in and wrenching his memories and feelings apart, and now maybe he thinks he's been doing the wrong thing this whole time even as long-honed paranoia whispers to him that now everything is definitely going to reset, now that he's said something entirely of his own free will.]

Nobody else should have to think about this.