I have no problems turning off a bad tv show. Or changing the channel on a boring movie. I've started to listen to an album only to realize there's only one decent song on it, and switched to something else. All of which I do without remorse. Books are another story. For some reason, putting down a bad book is hard to do.
Putting down a mediocre book is almost impossible.
I'm not sure why I feel guilty about not finishing a book, about taking the bookmark out when it's only half completed it's march to the last page. It seems to be much stronger when it's a book from the library. (Certainly there were books I was assigned to read that I put down without compunction, nevermore to pick them up again nor feel a twinge of regret for having done so. Even so, those were few and far between.) I think part of it is the idea that I picked this, I chose this particular book, so I owe it to myself to validate that selection by reading through it.
Sometimes I think it's a question of just the wrong book at the wrong time. There have been one or two books where the first time I checked them out I wound up returning them unfinished, only to get them again some time later and take them to completion. I don't often give books second chances. Usually it's only when I know it wasn't the fault of the story, or when it's a particular author whom I'm trying to give another redemptive shot to. In part this comes with the recognition that once I've put a book down from an author, I'm much less likely to get another one from them. (This has kept me reading authors who have long since managed to lose their spot on my "must read" list, by sheer hope that someday they'll pen something to find their way back onto that list.)
So I know, when I put a book down without finishing it, that author just got a black mark from me, and the odds of my getting another book from them have dwindled significantly. This means it is a major undertaking, a severing of either a well-established relationship, or the ending of what might have been a promising long term endeavor. I don't set a book down without consequences, and as a reader I tend not to be very forgiving.
Some of the guilt is also tied up with other people's expectations, especially if it's a story I'd heard good things about. Then it becomes a question of, everyone else loved this. I don't. Ergo there is something wrong with me, as a reader, that I don't get how awesome this is. It's not really a valid argument, I know, and speaks more of my own insecurities than anything else, but hey, we all have our neurotic ticks. This just happens to be one of mine.
Sometimes it's simply the reluctance to abandon a project once it's started, and often for reasons that make up only part of the whole. I'm finding myself struggling through Under the Dome right now, for example, because I absolutely cannot stand one of the major characters. I just want someone to put a bullet through his head, and suspect instead I am stuck with him for the next thousand pages or so. Abandoning the book now just because of one character feels slightly treasonous. Yet I have a hunch I may do so, and know also I'll check it back out again eventually.
Only to perhaps feel guilty all over again if I put it down a second time.