I think there's also another factor to this, if you don't mind me adding my two cents to the discussion. Sometimes, people can find enjoyment in things they don't enjoy, as bizarre as that sounds. For example, if you start reading something, and part of the way through, you find yourself no longer enjoying it, a part of you might want to finish just to say, "Well, I gave it a chance." Or if it's a popular series that you find yourself unable to get into, but know people are going to question you on it/your tastes, and you're the type that enjoys debates (or arguments, if you prefer that--not particularly my jam, but no judgment), then the slog of pushing through with the series now means you have "ammunition" to fire back with.
An admittedly silly example that I can think of was from years back, when the Twilight book series had blown up: I would routinely tell people that the books just aren't that well-written if you scrutinize them for more than a few seconds, and that while I would never tell anybody to stop enjoying them, I personally don't find them to be good books. Nine times out of ten, the reply would be, "Oh yeah? Well, I bet you didn't even read them, so you can't judge!" Let me tell you: I read those books cover to cover, petty as that was.
It's kind of like the spite example
@Kendama mentioned above: even if the act itself isn't enjoyable, there could be something else that comes from the act that, on some level, you find rewarding. I used to think I would have a completionist mentality, but as it turns out, I've already dropped a couple of series from my reading list, simply because--to me--there was no enjoyable outcome to be had from finishing. They weren't particularly popular series, I hadn't gotten deep enough into them to warrant the "I gave it a chance" mentality, and ultimately, nothing of value was gained
or lost in making that decision. It's sort of a case-by-case thing.
