To answer this question, we first need to understand why Via was upset. The day in question had been her first day of high school, and everything had seemed different between her and her two best friends, Miranda and Ella. This oddness bugs Via so much that she lies to Miranda and tells her that her mom is going to pick her up from school, when in fact she is supposed to get a ride with Miranda's mom. She winds up taking the subway home and lying to her mom about why she was late. She then has a fight with August, which makes her feel even worse.

I would argue that that Via's dad cheered her up by not falling for her story that her day had gone fine. He calls her out, telling her that he "didn't believe [her] for a second." He allows Via to open up and explain that Miranda and Ella had changed. He explains that high school is the ultimate test for any friendship.

He then lightens the mood by teasing Via about the fact that she was reading War and Peace. This reminds Via of how proud her father is of her, and of how she has heard him bragging about his fifteen-year-old Tolstoy-reading daughter.

Ultimately, Via's dad made her feel better by making her laugh. As Via wisely says, "sometimes that's all you need to feel better."