The Statistical Probability of Love at First Sight

"You know what they say," Dad said. "If you love something, set it free."

"What if it doesn't come back?"

"Some things do, some things don't," he said, reaching over to tweak her nose. "I'll always come back to you anyway."

"You don't light up," Hadley pointed out, but Dad only smiled.

"I do when I'm with you."
(page 123)

This quote shows how Hadley's relationship with her dad used to be.  They used to be very close when she was younger, and her parents were married.
 Now, Hadley has just seen him for the first time in over a year for his second wedding, and he lives a continent away.  Her dad left her and her mom and she was still upset with him for doing that.
Hadley's relationship with her dad got very strained because when she saw him it was right after the divorce, then two years later at his wedding.  That made Hadley feel like her dad had a whole new life without her, with the new wife and she knew almost nobody at the wedding.
Hadley "set her dad free" and by doing that became distant from her dad and even though her dad still loved her and was trying to keep in contact with her.  She also did that because she was angry with her dad for leaving her mom, and by doing so he also left Hadley.