"Readers have SHORT attention spans! The average reader takes just TWO sentences to decide whether to put a book down! You have to HOOK them in the FIRST sentence! GRAB them by throat and don't let them BREATHE—"
... have... have we considered that perhaps the average reader just, like, knows what they like in a book? I mean... first sentences are famous for establishing things like *checks notes*... genre, tone, POV, pacing, character, voice, uhhh... writing style...
The average reader is putting your book down because they discovered it's in first person (or not in first person). The average reader put your book down because they wanted a cozy read, or they're sick of cozy reads, or romance, or grimdark, or assassin princesses, or vampires, or or or. The average reader put your book down because they just didn't like your writing style—no, not because it was boring... they just, get this, didn't like it.
The average critical reader put your book down because it had six grammatical mistakes in the first two sentences.
The average reader will read quite a ways if the premise intrigues them, they like the genre, the writing style doesn't get on their nerves, and the characters pop off the page. In fact, they'll probably read the whole book, so long as it delivers on its plot promises and doesn't drag in the middle section.
The average reader will, however, stop reading after just two sentences if it's clear by the second sentence that the only thing they'll like about this book is the opening line.