Ok, so here's what I'm thinking - in at least three of the stories I have either had in my head or actually written I have killed off the main character's love interest. In another the main character's love interest got with one of her other friends. And then there's the one where the main characters were dinosaurs and one of them had to turn their whole species back into brainless reptiles, which is a long story but the point is that this meant he and his love interest were not intelligent enough to remember their love for each other. What I want to know is this - is an unhappy ending in regards to romance a good thing or am I overdoing it by doing it in THIS MANY stories?
Are you writing romance novels particularly? Are your stories designed to be read in succession? If the answer to both of those is no, then I fail to see a problem. You can be an author who simply doesn't write love stories or have tangential love subplots. Unless all your novels in a series keep killing off love then I don't see how anyone except you will spot a pattern and object to it. Personally I don't write love stories because I do not believe love exists between humans, at least not in the way it's depicted anyway. I experience love (unconditional, freely expressed without expectation) between me and my pets. I have never experienced anything close to that with another human...ever. I'm happy to write lust stories which I find far more credible between human's. That's my choice as an author. You have your choices to make as well.
It isn't romance novels particularly - it is more in the area of Science Fiction and Fantasy but I always like to embed romance into my stories. In fact, my main character doesn't even get a love interest in the story I am writing until the second of a series of four. But when he does she gets killed.
If you become a famous author, and ALL your books feature failed romances, I should think your readers would figure it out and get quite bored after a while. They'd stop connecting and investing in any romantic subplot. I won't pick up a book by an author if I already know they're going to use the same formula as they always have.
Using the same plot device again and again, regardless of the genre, is a double edged sword. You could make it a feature of your style, something readers will expect, and give them the payoff in varying other plot elements. Otherwise, it'll get old really soon.
Yes, but you're in the minority of book buyers. If you're a famous (and fabulously wealthy) author like, say, Janet Evanovich, you can just write the same damned book twenty-six times and your readers will snap them up.