[Helena makes a face when he dips his head, about to chide him that it'll be his fault if she cuts him, but he quickly corrects that--and says something that bothers her more.]

Yet again I'm reminded of how lucky I am not to have human brain chemistry. "Design flaw" is the wrong term, of course, but it feels like an apt description for what you're describing.

[Her razor-strokes get a little less gentle as she moves on to his cheeks--not truly threatening, but quicker, harsher, betraying her irritation even more strongly than her voice.]

I wonder if you end up being a little less diligent than you're claiming. Not on purpose, of course, but if that desire to "feel alive" has more sway over you than you realize.