Romilda jumped when Dennis snapped at her and looked down at her feet quickly. This is why she didn't want to talk to him. She didn't want him getting upset at her when all she wanted to do was make him feel better.
"Sorry," she mumbled. "You're not upset. And..." What was she supposed to say to that? Her favorite person told her he was worried about losing her mind and she wasn't supposed to say anything? That wouldn't do at all.
"I won't," she said, only because that's what he wanted to hear. But Romilda wasn't so sure she could keep herself from talking to someone.