He hates the Sun.
She tried arguing, once, that the Sun wasn't actually a Noldorin invention, that it rose after the Noldor'd left, that the reasons he had to hate the Noldor were not reasons to hate the Sun. That'd been years ago. After she'd noticed that he hated being argued with, before she'd realized that provoking him was not actually a good idea.
He hates the Sun and so the forest is sorcerously shrouded in darkness, but sometimes if she's sweet enough with him she can get permission to ride, and she can find spots where he was careless with his sorcery and she can blink longingly at things she thinks are sunbeams.