I don't usually start a book and not finish it. No matter how many times people tell me that it's okay to put a book down if I'm not loving it, I just have to. And I think part of it might be the need to find out how it ends, even if I really, really, really dislike some main characters. But mostly, it's because I have hope for the books.
I'm so optimistic about so many books. See, even if I'm reading a book that I complain about way too much, I'll finish it, because what if it picks up? And then, I'll casually mention that I read it, and somebody else will say, "Well, that book wasn't really for me." I'll say, "It wasn't for me, either! I just thought it might turn out to be for me."
So, books disappoint me sometimes. But I always feel this need to give them another chance, like maybe in the next chapter a character will make up for whatever choices were made in the last chapter.
However, it's not just my optimism. It's that I've been spoiled by too many fantastic books. For example, the books I read right after I read I'll Give You the Sun by Jandy Nelson were such a letdown, but not because they weren't good books--just because I'll Give You the Sun was a great book. (Even The Sky Is Everywhere, Jandy Nelson's first book, was kind of disappointing simply on the principle that it wasn't I'll Give You the Sun.)
So, I guess what I'm trying to say is that even when books disappoint me, it's (mostly) because I've had great experiences with books in the past. If those books can be great, why can't they all? I've been spoiled by books, and that's given me hope for all books to be as good as the others I've read. And, really, isn't that a great problem to have?