So, the series in question is urban fantasy/paranormal romance—the kind where the normal human population exists blissfully unaware of the Things That Go Nom in the Night.
Our Heroine is part supernatural creature, and haaaaates it. In fact, she has dedicated herself to hunting down and eradicating monsters like the one who gave her her non-human heritage. Of course, then the hero waltzes in, and he is entirely non-human. Cue mutual snarking, danger, combustible sexual chemistry, and improbable quantities of tight leather pants. At the end of book one, the heroine gets caught by (or at least caught up with) your friendly local Men in Black (ok, one guy in black, and a bunch of guys with guns. Really big guns) and offered a job. Possibly “offered” should be in quotes there. Besides which, she is panicked that the relationship isn’t going anywhere, because he’s essentially immortal and will never be able to accept that she’s going to age and die, while she’ll never accept the inevitable attempts to change her to being entirely supernatural (thus getting rid of her human side). Anyway….
Hero: At last! My years of searching for where the heck you fled to are over, and, by the way, you are not getting rid of me again.
Heroine: Ohnoes! You’ve found me! And you still look amazing, by the way… um… where was I? Right, the super-secret-supernatural-fighting government agency I work for is not going to like me wandering around humming “My boyfriend’s back” in the workplace.
Hero: Fine. Guess you guys just got a new addition to your team.
Heroine: But… but…
Hero: Look, I’m willing to go on an undercover mission in a BDSM club and gyrate around while wearing VERY slutty clothing that makes those trampy outfits you wore on stakeout in Book One look like winter convent fashion.
Heroine:… does that even technically count as clothing? Because from this viewpoint… look, never mind- the point is, government conspiracies aside, we can’t be together—you’ll never be able to accept that I’m going to grow old and die, and I am NEVER going to accept your inevitable attempts to get rid of my human side and change me over to being entirely supernatural.
Hero: Wait, what?
Heroine: I said, you’re never going to accept that I’m going to grow old and…
Hero: *snicker* You seriously think you’re going to grow old? With your bloodline? Honey, I don’t have any reason to change you into ANYTHING, especially because one of the easily-duplicated events from last book has pretty much doubled your lifespan already.
Heroine: How longed my what, now?
Hero: *sexy grin*
Now, may I just say, I felt a vast sense of relief when reading that, and cheered for the author, because it was a very simple, very plausible solution to the very annoying problem of what do you do in a paranormal romance where one partner is essentially immortal/very long-lived, and the other has a regular human lifespan—because too often the default is ‘well, clearly, make the other one immortal, and they will give up on being human, and it won’t really matter.” Quite frequently after infinite pages of meebling about it,
And then… and then… Book Four. Yes, four.
Hero: WHY WON’T YOU LET ME CHANGE YOU TO BE LIKE MEEEE? IS IT BECAUSE YOU SECRETLY STILL HATE YOUR NON-HUMAN NATURE? BECAUSE THAT PART OF YOU THAT SECRETLY HATES YOUR NON-HUMAN NATURE TOTALLY HATES ME? WHY DO YOU HATE ME?
And then, after this epic flipping out, he leaves.
Now, granted, at this point, there is a perfectly legitimate plot-related reason for him to fake a temper tantrum and storm out. Except… he never, not ever, not even once, says anything about it having been faked. Even when the heroine proceeds to rationalize her way into “Wow, gosh, I can see where he would be freaked out on my behalf, because really the only thing my human half gives me is the ability to get dramatically injured, and that totally wouldn’t be a problem if I were completely non-human, and if I love him, I really should do this for him…” and then tells him once he’s back, his reaction is not, “Um… wait, what? Because I was totally faking it for the hidden cameras, and didn’t we already have this discussion in Book Two?”
No, indeed. After a book where he quite explicitly stated that he didn’t care, and that her concerns were groundless, and a book where the issue wasn’t even glanced at sideways, all of a sudden it is the giant fangy elephant in the room. A room full of furniture the hero has tossed against the wall in an epic fit; seriously.
I mean… it would have been one thing if there had been a gradual set of problems developing—the whole “Well, I thought it was simply about lifespan, but it really isn’t, because I love you and your job is going to kill you, and the only thing that will save you is to either quit, which I would never ask, or to become something other than what you are, which I swore didn’t matter….” dilemma has a lot of potential. Not to mention that there was another potentially really interesting conflict where another supernatural race was concerned about the heroine choosing a different supernatural group to join, because that would give her additional special bonus powers of ruling the world (or something), and so they were demanding that she be forced to change over to the hero's type. Supernatural politics and scheming can be tons of fun, and the idea of "It's not your humanity that's causing problems, it's how exactly you plan on getting rid of it" had potential on top of the romantic conflict over the same issue.
But not when the whole thing (INCLUDING the cross-species political panic and smashing things all because of the heroine) arises out of nowhere, tears across the narrative, and then is resolved with the heroine having a paragraph and a half of beating herself up for having been so blind to her own residual anti-supernatural prejudices and selfish about wanting to stay partly human.
Not to mention the fact that when she DOES take the plunge, the crossing of her existing supernatural heritage with his results in BELLA SWAN VAMPIRES BETTER THAN YOU. Because of course there is no drama in her just leaving her half-human, half-nonhuman nature behind and settling easily into a fully supernatural existence. *sigh*