Well, he doesn't know! Maybe all fairies like to do that! Maybe it's a thing. Brock has been friends with vampires and elves, but he doesn't have much experience with the habits of fairies.

Also, he is not being a dick! He's just worried. He needs to keep her in a little jar (with airholes in the lid, natch) for her own safety.

Eventually, Brock notices the chandelier moving around, and looks up. "Ha!" he says triumphantly. "I see you."