Really, I don't.
But I did last Friday. It was a romance, and I had almost reached 50% when I gave up on it. The last straw isn't something I'm going to discuss, since I think it's specific enough that people might recognize the book, and I just don't do that.
I will, however share what had me laboring through a few hundred pages for eight freaking hours on a public holiday.
Conflict. See conflict to a writer is supposed to be something that keeps a main character from achieving his/her goal. When it comes to pure romance, it's about what's keeping the characters apart. Sometimes, it's something like either the hero (MMC) or heroine (FMC) being engaged, or them wanting opposite things in life, or one just not possibly imagining that the other could be a suitable spouse/partner/whatever.
With the latter, it's usually about one or both of the characters being magnificent assholes/bitches. (Think Jack Nicholson in Something's Gotta Give or Sandra Bullock in The Proposal. Or my personal favorite: Harrison Ford in Sabrina.)
Now rule of thumb is that the more hard-assed and untameable the character, the sweeter the happy ending. And time and time again I have seen people simply taking this rule at face value and abusing it. Which often takes the form of the "Happy Ending" being with someone who's either almost or fully abusive. This is particularly prevalent in the falling for the alpha male trope.
The argument could be made of "what message is this sending to the reader", but as is very well documented on my blog, I don't believe in moral preaching in my writing. As such I won't expect it from others.
I do, however have a major issue with writers abusing that rule for one season and one reason alone:
Suspension of disbelief.Any fictional story, no matter how realistically written, requires for a reader to suspend disbelief. With romance, this is incredibly important because the reader must want to believe that two characters will be together. Because unlike most other genres, this goal is usually not decided on by the characters. (The opposite, in fact.) It's all in the reader's mind.
So if one of the characters in the prospective couple is an asshole (since I mentioned the alpha male, the character will be male. This is the same for female characters too, though), the writer has an additional problem. She/he will have to engage people in the asshole enough for the readers to want him to end up with the FMC. And then, the readers must believe that the FMC would be happy with him.
This can be done in a variety of ways. First, by showing the reader that there are other sides to him. That there's more to him than the hard-assed exterior. (And even if there is, nothing excuses him from remaining an ass towards the FMC in the end. I repeat: NOTHING.)
The second (and I admit a preferable way) is for the character to go through a growth arc before the get-together in the end. Note the three movies I mentioned above all have this happening.
But in no shape or form is half-way the place to start with this. If it's half-way into a romance and I as the reader would reverse over a character if I hypothetically hit him/her with a car, there's something seriously wrong. And if I get to the end and the character has nothing redeeming him (hot sex doesn't count), the writer of that book has essentially betrayed the trust required for suspension of disbelief. Because 1) I don't want the FMC to spend the rest of her life with and asshole because 2) I can't imagine her life being happy for long because of it.
So please please please, romance authors. Throw us readers a bone. Let us actually like the characters as much as you do?