Sollux knows his decision upset Charles deeply. He also knows that without it none of them would be alive - none of them would have the safe haven from Hypatia that they do now. Someday the AI will be gone, Sollux has faith, and then he can be unplugged - then he can be more than a disembodied voice again. But right now, they need his head more than the rest of him, so this is what he has to do.

He "smiles," his mental presence cozying up to Charles'. You haven't read to me in a week, Dad. I miss it. Sollux knows that Charles hates visiting his tank, having to be in the presence of his own adoptive child suspended in sensory deprivation; but he craves the interaction, these days. With his mind fully capable of doing millions of calculations per second, he's a lot more aware of the time when he's alone.