Some of it also comes from prior impressions that a book is "worth reading". Take Wealth of Nations as an example. It inspired, arguably, the whole field of economics - and yet after a hundred or so pages of reading about the worth of the labour of a man in Glasgow as compared to the labour of a man in London, I just wanted someone to end my misery.

Alas, I'm afflicted by the "must finish" disease, so I paused reading and keep telling myself I'll get back to it.