It may be nothing less than a compulsion. Like the hunger of Ghouls, or of the wraiths.

[Still, even as he says it it seems obvious that the explanation sits poorly with Celehar. There have been flashes of something else, from the ghoul in Tanvero, and from Shepherd's report of the ghoul whom she'd spoken to.

Or maybe it is, as Need has correctly evaluated, the fact that the experience has rattled him. He does his best to adjust, when she points it out, straightening up and glancing away.]

It felt dreamlike, but I would hope that none repeat the experience.