So I noticed this during my final Pacifist Run...
Here’s Sans as a Lost Soul. Notice anything unusual about his speech text? Let’s compare to this GIF of Alphys.
And here’s a close-up on his speech.
It’s hard to tell when it’s not in motion, but the letters are shaking. No other Lost Soul does this. Not even he does this at any other time; not even when you face him in Genocide Mode. His text never shakes like this at any other point in the game. And what does he say? “just give up. i did.” “why even try?” “you’ll never see ‘em again.” When Sans faces you as a Lost Soul, he can’t hide the emotion in his voice any more. Killing his brother won’t get this reaction, being killed by you won’t get this reaction, because to stop himself from being hurt any more, Sans has made himself numb. Even if you’re Genocidal and you commit atrocities like he’s never seen, he’s too far gone, so deep in despair that he’s dead inside. But if you play pacifist and befriend Sans? He sees you tread the same path as he did, trying to make things right despite overwhelming odds. He looks at you, and he sees the innocence he’s lost. And suddenly he’s out of Snowdin, he’s following you across the entire Underground, and he’s rooting for you, kid. You’ve given him hope. You’ve given him a reason to break out of his rut and actually care about something, and that something is you. Behind that glitchy mess as a Lost Soul, he’s crying. And given this is the guy who’s disguised his pain with laughs for the entire run, that’s a pretty huge deal.