Sometimes I don't finish books because I've become someone else in the interim and don't share their goal of reading it. Any book that takes more than a week is liable to fall foul of young people’s mutability in this way.
A lot of what we read is just to say we’ve read it - they are plugs for gaps in cultural armour. We fail to see these through because they are interminable – cf. Gibbons’ Decline and fucking Fall – and because our motive’s so base in the first place. The act of plugging could be noble – the will to improve oneself – but it's more often the ignoble fear of looking ignorant (rather than the excellent fear of being ignorant). The educated world keeps up an arms race in which indifferent bystanders are gunned down by fully-auto sneering, where books are secondary to the concept of themselves. This side of ‘literary’ culture, call it the consumerism of the immaterial, is scarcely different from more obvious consumerism about designer labels and very large cars. Each of the games motivates the player with identity concerns, providing us with superficial status by association and not via anything actual like form or content.
Relatedly: we can stop reading out of simple disappointment (because misled by hype) or simple disgust (because of unbearable prose). I’m no aesthete, but on occasion I’ve had to bail out because of style; bad translations from e.g. Swedish or Chinese, academic work of any field from the last 50 years.
Finally there are books you haven’t finished reading even when you’ve read to the last page. The hermetic, or meaningless, or stodgy, or countably infinite: Finnegan’s Wake, Pound’s Cantos, Moby Dick, Infinite Jest. Whether it’s worth returning depends on how poncey your peers are, or how much you trust the book’s ineffable reputation (which will be to the same extent you distrust your ability to actually read).