The troll glances back to the dog, partly because it's caught his attention again, but mostly because he doesn't like seeing the way San's smiles when he says something so grim. Of course you can't change the past, but he's pretty sure that isn't the point Sans is trying to make here.

"Most people like to think they can change things, is probably the problem."

Agency; it's something he's thought about a lot. Back home it had been something he struggled with and something he'd been fighting for. Here things were different. That's what he liked to believe, at least. He didn't have much choice about what happened or where they went, but he could at least choose to stay or go and conduct his work in whichever manner he thought logical. It was a better life than that of a slave, even if technically he still was one.