[Pitchwife]

I may be mistaken, but I've always had the impression that Nellas was, by Elvish measures, still a child, or at least a very young girl - like, if she'd been human, she'd be no older than thirteen or thereabout; her initial shyness before Thingol seems to me like she wasn't only intimidated by being in the presence of the King, in a company of strangers in a a big stone hall to which she was unaccustomed, but rather embarrassed like a child being questioned by adults. Then, once she's gathered some courage to speak, she has the naïveté to go babbling about Lúthien sitting in a tree, of all things, when every adult resident of Doriath must have known that this had to be a somewhat sensitive matter for Thingol!

No doubt she was still wise far beyond a Mannish boy of comparable age, and there was much that Túrin could learn from her; but he matured at a much faster pace than she did and so outgrew the desire for her company. And if there's one thing a teenage boy in the throes of puberty will avoid like the plague, it's being seen talking and playing with little girls! From this angle, it all makes sense to me.