Derek sort of sees Sakura like a little sister that needs to be protected. He's relieved that she is dating Xiaolang because she has a tendency to get into trouble (not her fault), so in Derek's mind Xiaolang and keep a good eye on her. He thinks she's pretty naive to the more difficult things in the world. Part of him wants to inform of such things, while he also kind of wants to protect the innocence she portrays.

Either way, Derek feels protective of her. Mainly (but not limited to) because she is part of this really cute relationship that Derek admires.

He still doesn't really get her magic, but he also hasn't seen a lot of it.