Jenny Fields: You know, everybody dies. My parents died. Your father died. Everybody dies. I'm going to die too. So will you. The thing is, to have a life before we die. It can be a real adventure having a life.
T. S. Garp: We'll take the house. Honey, the chances of another plane hitting this house are astronomical. It's been pre-disastered. We're going to be safe here."
Jenny Fields: Are you gonna go to sleep or you gonna stay up and think your weird thoughts?
Young Garp: I'll stay up and think weird thoughts for a while.
T. S. Garp: Men die young in my family, Helen.
Answer: The book makes it pretty obvious that there are a number of factors: 1) She caught Garp and her sister in the middle of things. 2) Garp hurt her dog. 3) She actually hated him from the start and it just got worse.