By the end of Paul Zindel's novel The Pigman, the two protagonists, John Conlan and Lorraine Jensen, have realized that their greatest problem is their immaturity and that it is their immaturity making their lives what they are.
Both John and Lorraine feel alone and alienated by their families. John feels his parents refuse to understand him, making him feel bored with the world and act out by pulling pranks. Lorraine was abandoned by her father and rarely sees her hardworking mother. John's invented prank to telephone strangers leads the two characters to meet Angelo Pignati, who is equally lonely due to his wife's death.
The three develop a close friendship in which they all influence each other. John and Lorraine even influence Mr. Pignati into roller-skating inside of his house, strenuous exercise that gives Mr. Pignati a heart attack. While he's away at the hospital, due to their immaturity, John and Lorraine further take advantage of Mr. Pignati by throwing a party that results in many of his things being wrecked, including his wife's collection of porcelain pigs. Learning that John and Lorraine, whom he loved and trusted, actually cannot be trusted proves to be too much of a strain on him, and he dies of a second heart attack.
Sadly, had John and Lorraine behaved in a much more responsible, mature way, they would not have caused Mr. Pignati's death. Hence, they realize that it is really their own immaturity making their lives miserable, nothing else. As John phrases it at the end of the book, "Our life would be what we made of it--nothing more, nothing less."