Maybe it's that I've read a lot of books. Maybe it's that I have developed preferences (not to say 'taste') and am comfortable defending those. Certainly it's in my willingness to take on a controversial position on aesthetic matters. But I cannot imagine the circumstances under which I would pretend to have read something I have not.
I may have read 1000ish pages of it in one night, but I read all of War and Peace. I may have felt vaguely ashamed to not have read Moby Dick, and I certainly have regretted feeling ashamed. I sometimes, but less frequently now, finish books whose quality is uneven.
I haven't read Nabokov, not even the ostensibly in-my-wheelhouse Pale Fire, not least because having to read the poem to get to the commentary is not among my interests. I've read a little of his criticism, which alternates between perceptive and insane, and the manner in which the novelists I like mention him inclines me to think he's not worth the time. I thought the same about Henry James, though I think I am beginning to come into the right time of life to read him (I made it halfway through The Turn of the Screw last year before having to discard it for end-of-semester concerns). I sat down to read Don Quixote this summer, made it through two chapters, maybe three, before realizing the time in my life to have read it had passed ("oh. this is just going to be a very long, very intricate parody."*). At 600 pages into Infinite Jest DFW was introducing another new character and great length, and I bailed. Conversation at the Cathedral (the first time) when I realized that five separate conversations were happening simultaneously in the narrative.
I suspect a list of books I'd abandoned would be just as interesting as a list of books I'd finished.
I go through certain phases where authors are better appreciated or neglected. I take this to be natural, and not worth the effort to fight against.
*Lest I be accused of snobbery, this was also my reaction to Beauvard and Pécuchet, which at least varied the thing parodied, and had a few deeper points buried in the narrative. Made it halfway and then quit.