It's amazing the range of emotion Sasuke could manage while still wearing that bizarrely cheerful smile (or variations of it). She'd come to the conclusion a little while back that he'd been caught by some sort of jutsu (ninjutsu, she presumed, as he would have overwhelmed any genjutsu by now) and that explained why it was stuck like that.
While he was being stubborn, though, there wasn't much she could do to help him.
It was actually kind of cute, the smile he wore when he wasn't projecting murderous intent at anyone who he caught looking at him. So, basically it was whenever she happened to look at him while he was alone.
At some point, she noticed a lack of it while they were sitting together drinking tea. She wondered if she should be flattered by that or not. He still didn't ask or allow for the possibility of her freeing him from the jutsu, but she kept an eye on him for signs of injury all the same.
Ultimately, that was her explanation why, when they were sitting in a secluded place without much conversation (which was simply normal when sitting with him, as he tended not to speak much without purpose anyway), she leaned in and placed a light kiss to the corner of his lips.
With the kiss was healing chakra. Nothing that could fix him without knowing what was wrong…but it would soothe the muscles that had been forced to hold his expression like that so long. "…You looked…sore."