"No one's positive about anything, when it comes to you."

--Compliment, or?? Could be. Could also be that he's politely suggesting she's out of her mind.

"Desperately," he echoes as he finally moves, walking past her in almost a glide, surprisingly graceful for someone who looks so distinctly unfortunate. "Wouldn't you like that."

Severus slips past her out of the common room.