It may be partly because I don't have the time left, in years, anyway, to read all the books I would like to read, or the patience to struggle through the ones that seem to be written in Exclamation Points or sludge.
Some years back I realized that a book that hasn't gotten my attention by the end of the first chapter is probably not going to be worth the walk through the rest. I was reading a book at that point in which the hero "shirted and panted himself" (now there's an image to take with you) and this arch style of writing--which seems to have foretold the advent of the verbing of America--so annoyed me I closed the book and took it back to the library. It took me four tries before I finally understood that I will never read "Vanity Fair", no matter how I try. I have started and abandoned "Hard Times" at least that many times, and even though I have read and enjoyed most of Dickens, for some reason i always lose interest at exactly the same spot in this book, every time.
The idea has always been planted in our heads that these authors worked very hard on this stuff, and it's almost sacriligeous to start a book and never finish it. Like taking two bites out of a piece of cake and then throwing it away. It's the equivalent of the Clean Plate Club, which only fosters guilt and fat. And once I understood that no one was going to suffer if I didn't finish a truly dull book, and someone would if I did (me, usually), life got just a tad simpler.
I will admit to a pang of guilt when someone loans or gives me a book that they love, and I can't even wade through the first chapter of it, but that passes. One truly disappointing book was Barbara Kingsolver's "Prodigal Summer", which, by the end of the first chapter had degenerated into a cross between "Girl of the Limberlost" and "Lady Chatterley's Lover", and since it also was turning out to be a Save the Animal Story (which I cannot abide), I quietly retired it to the giveaway bag.
Life is too short. Read what you like.