Maybe, though I'm not sure if it was ever the case, when I was younger I used to think I ought to finish a book if I started it. That somehow I had entered into some kind of social contract, whether with the author or with the library I borrowed it from (since most of the books I read till I was in my 20's came from that source) and it would be rude not to.
But life is too short to read books you don't give a crap about and there are always plenty more waiting to be read.
Anyway, on to talking about why I don't finish some books that I start. The common factor seems to be this - that I don't care what happens. It could be that I am so underwhelmed with the characterisation that I have zero empathy for the protagonist and their struggles. It could be, though this is less common, that the plot is so dull I literally can't summon up any enthusiasm for finding out what happens in the end.
Those are the main two killers for me. I can tolerate a lot of nonsense, getting things wrong (either blatantly or subtly) for whatever reason - a recent book had all sorts of social niceties being ignored by the author and, if I had cared enough about any of the characters and their eventual fate, I could have glossed over that. Heaven knows I've read enough murder mysteries written by American authors set in a pastiche of England to evidence that particular theory. But I didn't, so I couldn't.
I'm sure there are other things that make me go 'nope, let's not' but those are the most obvious and, sadly, the most common. Anyone else got something that makes them go 'nope!' and hit reverse?