Do I finish every book I start? Ha ha ha. No.
In fairness, it depends on your definition of “start a book”.
I’ll often read the first few pages of a book in a bookstore or library (or the Kindle sample) to decide if I want to read the book or not. I don’t count this as starting a book. It’s part of the purchase decision.
I have the same attitude to books I’ve borrowed or got for free (e.g. as a free ebook download). If I haven’t paid for the book, I’m not invested in any way and don’t feel any obligation to finish the book I’m not enjoying if I haven’t paid for it. In these cases, I treat “starting the book” as the same as the Kindle sample: it’s part of the purchase (or reading) decision.
I’m much more likely to try and finish a book I’ve bought, because it’s cost me real money. Even though I check books out before I buy, I do occasionally find I’m not enjoying a book I’ve bought. In this case, I will put it aside and come back to it later.
Sometimes I enjoy the book the second time I try. Sometimes I don’t.
But even a paperback only gets three chances before I give up. There’s one book I bought by an author many readers love, but I’ve tried three times and jsut couldn’t get into it. I read another one of their books and still couldn’t see what everyone was raving about, so I don’t think I’ll try again.
I almost always finish review copies, because that’s the point.
I was given a free copy with the understanding I’d read and review. Even then, I do come across the odd book I can’t force myself to finish.
My did-not-finish books are almost always because of the characters. It’s hard to read a book where I don’t like one (or both) of the main characters. Sometimes that’s about the character being intentionally unlikeable, but it’s more often because the character says or does something stupid or inappropriate, and that’s not portrayed as an issue.
For example, if a character makes a sexist or racist comment and another character challenges them, that’s okay. If the comment is ignored, I wonder why … and if the author, editor, and publisher all think that’s okay. (This is why it can be useful to have an author’s note at the beginning, so I know to expect inappropriate language and know why it’s there).
