The first paragraph of the novel primarily serves as
"If you really want to hear about it, the first thing you’ll probably want to know is where I was born, and what my lousy childhood was like, and how my parents were occupied and all before they had me, and all that David Copperfield kind of crap, but I don’t feel like going into it, if you want to know the truth. In the first place, that stuff bores me, and in the second place, my parents would have two hemorrhages apiece if I told anything pretty personal about them. They’re quite touchy about anything like that, especially my father. They’re nice and all - I’m not saying that - but they’re also touchy as hell. Besides, I’m not going to tell you my whole goodam autobiography or anything. I’ll just tell you about this madman stuff that happened to me last Christmas just before I got pretty run-down and had to come out and take it easy..."
To shock and alienate the reader
To create sympathy for the narrator's unhappy childhood experiences
To give hints about the narrator's current circumstances
As an objective assessment of the narrator's character