The book's opening "oath" tells readers a bit about the answer to this question:
Being of sound mind and body on this 15th day of April in our sophomore year at Franklin High School, let it be known that Lorraine Jensen and John Conlan have decided to record the facts, and only the facts about our experiences with Mr. Angelo Pignati.
John and Lorraine finish the book's opening oath by telling readers that nothing but the truth is being recorded in the following pages. We have no idea who John, Lorraine, or Mr. Pignati is at this point, but the first two chapters of the book then inform readers that John and Lorraine are writing the book's very pages for two general reasons. First, the book is somewhat of a tribute to Mr. Pignati and the relationship that the two protagonists established with him. The second reason for writing the book is that the writing process is essentially a coping mechanism to help John and Lorraine deal with their feelings about Mr. Pignati, their experiences with him, and his death. Lorraine's first chapter (chapter 2) flat out tells readers that she and John feel compelled to write down the events of the last few months. They want to write them down while the details are fresh and before they decide to repress all of those emotions.
It’s just that some very strange things have happened to us during the last few months, and we feel we should write them down while they’re fresh in our minds. It’s got to be written now before John and I mature and repress the whole thing.