The boys’ obsession with Cecilia (and the Lisbon sisters in general) starts to really take shape in this section, as it goes from a somewhat normal—albeit exaggerated—fascination to something a bit more desperate. This desperation most likely has to do with the fact that they were there when Cecilia killed herself; they feel involved in the tragedy, and now they want to do whatever they can to find out
why Cecilia decided to die by suicide, even if this means invading her personal privacy and stealing her diary from her grieving family.