[quote=Kaga]
I would assume that she can recognize a smile on someone else, though I don't understand how, seeing as how she doesn't understand a smile as I describe it to her.Like, she was surprised to hear that you're supposed to show your teeth. She said she thought that "showing your teeth is a sign of aggression". And I was like, "Well, yes, it is... among chimps, and probably some other ape species, but not among humans, generally."She then said that the reason she thought that showing your teeth implied aggression was because, in her hometown, she used to hang out with "vampire people" who would hiss and "bare their fangs" at people. She then demonstrated what this looked like. I then told her that while teeth are involved in that gesture, 
[/quote]

It's probably the teeth thing.
A lot of people don't show their teeth when they smile, especially if more reserved by nature (or simply reserved when on the spot, or feel as if they're imposing too much).
Like most of the time when I smile I don't show my teeth, you have to be making me laugh for my teeth to show (Or I'm purposely making a very creepy/disturbing smile).

[img]http://i644.photobucket.com/albums/uu167/GameAddictX360/temp-320-75286329.gif[/img]

And being raised around odd people such as "vampire people" tends to produce odd people.
And if showing teeth was something she learned culturally to associate with a negative, it's no surprise that she's reluctant to show it. Or associate it with something positive.
Though, I'm simply trying to find a rational/explanation for Henry's reasoning here. I fully admit Henry's off, and therefore no defense I can give is bound to make perfect sense. 
Except to Henry maybe.