The smile is one thing but look at her eyebrows. More of a gentler smile with them like that.
Actually, the eyebrows have exactly the opposite effect on me. Eyebrows with such a curve are also used for psychotic, unstable expressions in e.g. anime. I could give you some examples.
I wonder how does one stay smiling when having sex?
I mean, both protagonists of Sun and Moon just smiles all time (except at some parts)...
Best explanation I've seen is that Sun/Moon don't actually speak a word of Alolan, and are just smiling politely at every incomprehensible thing they hear, and make a facial expression when they actually figure out from context what's going on.
In that case, sex is nonverbal enough to figure out and get the proper expression.