Loneliness drives people into bad situations
Pignati is so isolated after his wife's death that he welcomes two strangers who are clearly lying to him. John and Lorraine are lonely too, just in different ways. That shared loneliness is what pulls them together and what makes the damage so bad when it falls apart.
Adults fail teenagers, and teenagers fail adults
John's father is emotionally absent. Lorraine's mother is bitter and controlling. Neither teen has a reliable adult in their corner. Pignati fills that gap, but John and Lorraine are not equipped to handle the responsibility that comes with a real relationship. Everyone lets everyone down.
Guilt cannot be escaped by blaming someone else
Both narrators spend much of the book pointing at the other one. By the end, the novel makes clear that blame-shifting does not work. They both made choices. The memorial format is their way of finally sitting with that.
Childhood and adulthood pull against each other
Pignati acts more like a kid than the teenagers do sometimes. He loves the zoo, roller skates, and games. John and Lorraine are being pushed toward adult responsibilities they do not want. The three of them create a space where none of that pressure exists, until reality crashes back in.
Trust, once broken, cannot be repaired
The relationship between John, Lorraine, and Pignati is built on a lie. Even when real affection grows, the foundation is cracked. The party does not just damage the house. It makes visible what was always underneath.