How we cite our quotes: (Chapter.Paragraph)
Quote #7
Dan gave the backpack another tug, but it wouldn't budge. He looked at the train, and Amy saw he was crying. She didn't understand why. (16.125)
Dan's falling into a stereotypically male trap of not talking about his feelings. Or maybe it's just because he's eleven. In any case, he doesn't actually talk to Amy about their parents. Maybe if he did share his memories, losing the photo in his backpack wouldn't be as big of a sacrifice.
Quote #8
Maybe [the photo] had been ripped to shreds, or maybe [Dan's] parents would be smiling in the darkness forever with no company but the rats. (17.2)
The photo of his parents is the one sentimental thing he brings on his adventure, and now Dan's lost that. At this point, he's pretty much given up everything he holds dear for this clue hunt.
Quote #9
[Amy] held out the glowing green vial. Ian took it and she snatched the antidote out of his hand. (18.76)
This glowing green vial is a valuable piece of the 39 clues puzzle, but Amy has to give it up to save her brother. Even though the hunt is a higher priority to Amy than money, this shows that her brother is number one in her mind. We doubt Ian and Natalie Kabra would do the same for one another.