She thinks that this is maybe the silliest thing she's ever done {and she's done quite a lot of utterly silly things} because she's standing in front of her snarky Potions professor, staring at him and he's staring back. And the eye contact is more than she can take so she looks away and oh look, the dungeon is infested with Kraggles.
He clears his throat impatiently and asks her what she is doing all the way down here and why isn't she celebrating with her friends because surely that is what she should be doing on her last night in the castle. Of course she can't agree more, she really should be talking and laughing and remembering but instead she is several floors downdowndown in the cold dungeon. She smiles a bit and says that there is nowhere else that she'd rather be {and she thinks that she might actually be telling the truth} though he must not believe her because he frowns and tells her that her sarcasm does not amuse him. She shakes her head, because nonono that wasn't what she meant and she frowns with him because he won't ever understand.
But why would he? He is an ex-Death Eater and she is just a silly little girl that has a crush on her teacher. But there's no way that she'd be able to leave forever without telling him at least once what those silver eyes of hers saw in him all those years ago. She could see how much he was hurting inside and how lonely he was behind all those insults that he put between himself and others like a great iron shield. And all she had ever wanted to do was to be the one who got past them and figured out who he really was because she had a feeling that that person would be different than the man who stood scowling before her now {she didn't even care if she never got the chance because she just wanted to tell him}.
But now he's cleared his throat again and has started to look slightly uncomfortable. And she smiles the most encouraging smile that she can muster and says that she's always admired him and that she really is grateful for all that he's done for her and everyone else. Behind that mask she can tell that he's surprised even though many people have already thanked him with more creative words than hers {but she thinks that maybe he is surprised every time}.
Then she asks if she can write to him sometime and she definitely thinks that he'll say noofcoursenotnever. But he doesn't. He just looks at her like he can't understand what she's trying to do and maybe he can't because he hasn't had many friends {especially ones that used to be his silly little students}.
But then he tells her how strange she is, like she wasn't already aware. And he almost smiles {she can tell because the right corner of his mouth twitches ohsoslightly}. And then he nods his head and she wants to scream and laugh and dance in circles but she doesn't.
{Because that would just be silly.}