The book I’m reading right now is a ghost story. (It isn’t the MG horror I mentioned before. That’ll probably be next.) It’s not a kids’ book.
It’s a ghost story where the main character is a doctor who doesn’t believe in ghosts, which is fine. It’s probably best to have an MC for a ghost story who starts out not believing in ghosts. But it frustrates me as I’m reading the book (I’m about 4/5 of the way through), how he still won’t even consider the possibility of a ghost even with a lot of evidence and other explanations ruled out. It’s good that he needs to be convinced, but if he insults the victims by questioning their sanity (in the face of other witnesses and evidence) before admitting there might be something supernatural going on makes me want to reach inside the book and whack him upside the head.
In most cases, I don’t think it’s a good idea to make your reader want to smack your main character. I don’t like it here, but it’s excusable partly because it’s late in the book. I don’t think many people will stop reading when they’ve already read this far, though they might not recommend the book to others. And partly because it’s consistent with what the main character is like throughout the book. That much is good.
Have you ever wanted to smack the character in a book you’re reading? Do you think it can be a good thing sometimes?
lightingliramor 
This is a very interesting question. In general, no, I don’t think it’s a good thing, because it often means the character has lost the sympathy of the reader. It’s impossible to bring ALL readers along with you, but if you alienate even a sizable minority, it might be an indication that, as a writer, you haven’t successfully conveyed the character’s perspective. However, I think there are some characters who are created to provoke, and in that case, the smack-worthiness may have been the writer’s intention. Not sure which is the case in the story you’re talking about, but I agree that the urge to smack is more off-putting when it’s persistent even into the final stretch of the book.
I agree about losing the reader’s sympathy. I can’t tell if the author did this on purpose or not. The smack-worthiness* wasn’t really persistent to the final stretch because it didn’t start until some point past half way when the doctor was seeing clear evidence.
(*that is now an official term.)