Robin opens his mouth to speak—and then stops, waiting for you to decide whether to share it.
Even though you've spent the evening looking for something to help Robin, now that you've found this diary, you don't think you can handle the idea of it leaving this house. It was always meant to be secret, and you're not sure you have the right to decide otherwise.
Still—you can't resist your own curiosity about whatever's in there.
You can see the disappointment in Robin's face when you tell him you want to keep the diary private—but it's nothing compared to the longing he seems to feel just at the prospect of reading it. Quickly, as if not wanting you to change your mind, he nods