"I'm fine." It's a polite fiction meaning "Please, let me cry in peace." It's all he ever says, anymore, the only thing he will allow to break his mourning. And there is a lot of mourning to be done, for everybody who died in the war: Cedric, Cho, Ginny. For Dumbledore, killed by Draco Malfoy; and for Draco, killed by Lucious Malfoy. For Neville, given to Bellatrix LeStrange as a pet. He's fine, too.
He walks through Hogwarts some nights, when his tears have dried and he cannot sleep. The halls are quiet now, with so many gone. Even the portraits don't talk like they used to, but stare out of haunted eyes, and whisper the same few words. "Hush, children, it will all come to rights." The children are all gone now, but that doesn't matter. He thinks it's a lot of words for saying "I'm fine."
He realizes, dimly, that he has become the laughingstock of his peers, that they have been using fine to mean crazy, but he doesn't care. They could well be right. He says it, testing the feel. "I'm fine. I'm fine."
The Great Hall has become a bit of a meeting place among the survivors, and they have taken him to be a bit of a mascot. In a post-war world that's as nobody had hoped, a bit is the most anybody can manage. Lucious Malfoy looks up, his eyes only a little bit haunted.
"Hello Severus. How are you?" The smile is either pitying or mocking, but he's not sure which. It doesn't matter, the answer is the same.
"I'm fine."
They smile again, so today it was pity. They've been mourning their own losses, then. He doesn't, for all that he lives here, consider them his. Severus is here for one man only.
He moves on. No one tries to stop him, no one touches him. They learned early on not to touch him. Spells aren't words, at least in his mind, and he'd crucio'd the last person to touch him. They find it simpler not to try, these days. There are other, more important thing to do, and while Severus stalks the halls of the school-turned-palace, they are out re-building the world. Creating a new world.
Besides, it is unwise to touch that which belongs to your master, especially a master as unpredictable as the Dark Lord has proven to be.
He keeps walking. He ends up, as he always does, at the head of the Great Hall, where Dumbledore once sat. Ends up at the right hand of the man who killed thousands. As he always does, he finds himself by the side of the only person who can touch him; the only person less fine than he is.
"I'm fine," he says, less a greeting than an announcement of his presence.
"You're here," says the Dark Lord in return.
And Severus basks in the presence of the only person who understands his grief.
The Dark Lord, Harry Potter.