Because getting the right opening sets the whole tone for the book--for the author getting started, as well as the reader...

Here's the start of The Haunting of Hill House, by Shirley Jackson:

"No live organism can continue for long to exist sanely under conditions of absolute reality; even larks and katydids are supposed, by some, to dream. Hill House, not sane, stood by itself against its hills, holding darkness within; it had stood so for eighty years and might stand for eighty more. Within, walls continued upright, bricks met neatly, floors were firm, and doors were sensibly shut; silence lay steadily against the wood and stone of Hill House, and whatever walked there, walked alone."

Why it works-- This is three sentences, almost dreamlike, each full of images connected by your favorite and mine, the sneaky and knowing semi-colon. Semi-colons warn you that the sentence isn't done, there's more to come, and with sentences that build a sense of deep foreboding, as these do, it works beautifully. They're also, to me, sentences that 'sing'; you can hear a voice behind this paragraph. I hear a woman's voice, calm and uninflected, although maybe it's female only because I know the author is female--but I'd like to think it would be an offstage female narrator, either way. In my head, that suits this tone very well.

Nothing can live for long, and remain sane, without dreaming. "Hill House, not sane", does not dream. It holds within its walls a reality that must, by definition here, be insane. Also note the phrasing "not sane" vs "insane". The former is colder, inhuman almost, a nice contrast to the idea of the house "holding darkness within" like a living thing, a person (not sane) keeping secrets hidden. And note how the house stands "against its hills" -- "its hills", not the hills. This is a well-defended house, ready, prepared for anything. Just with that simple description, I'm already getting nervous.

"Within, walls continued upright, bricks met neatly, floors were firm, and doors were sensibly shut" continues with the idea of this place in a defensive posture, under control, unquestionably in a condition of "absolute reality"; and thus, not sane.

"Sensibly shut" -- I love this double-edged description. Sensible: yes, of course. Who could quibble? Let's keep the drafts down, let's muffle the noise, let's hide what we don't want others to see, or know. A house with mysterious, awful things to hide.

"Silence lay steadily against the wood and stone..." like a living blanket capable of smothering anything that would disturb it? Or maybe more tightly, like a shroud...

"Whatever walked there, walked alone." -- Self-explanatory.

Sigh. Lovely.

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Oh, and, for my "Stella Wars" cohort: remember those pesky Merekats? The main character of Jackson's We Have Always Lived in the Castle is named "Merricat". I know I read that book in high school. So that's another name nicked from long-past reading, just like good old Hissarlik.