"You're both stupid," Hermione snapped. Ginny more so at that point, but Ron was there so he was going to bear the brunt of her anger. She was still tightly wound from her day of worrying and it was just bad luck that there happened to be a catalyst to set her off.
"What Ginny did was completely ridiculous and I think she's starting to realise that, but you're eighteen years old, Ron, not eight! What do you think throwing your journal away is going to achieve?"