I suspect a lot of writers, or at least a lot of genre writers, have some form of sequelitis. After all, so many genre novels are parts of series. Benign sequelitis, like I have, has got to be the best kind, for the reasons I mentioned, and because it keeps any sequels fresh and exciting. But there's also
- induced sequelitis — Publisher: Your book's selling great! Can we have three more by Christmas? Hollywood has an epidemic of this at the moment.
- malignant sequelitis — Writer: I'm writing urban fantasy, therefore I must create series. Often leads to overstuffed worlds or writer's block.
- galloping sequelitis — Writer: I have such a following for my book, I really owe everyone a sequel. Christopher Tolkien and Brian Herbert have had this, I think, with a milder form of induced sequelitis as well.
And yes, you can argue that I'm biased towards the benign form because I've seen too many books (and films) produced by induced, malignant, and galloping strains of sequelitis that have been subpar or rushed. I'm sure a writer who really knows their game can craft excellent demanded sequels and not fall into the trap of repetition. I'm sure I've read some of those books. Can I name them after being up till 3 last night? Nope. Not the writers' fault, though. Totally mine.
But really, I think that's the cure, that's how to turn the negative forms of sequelitis back into the benign one—step back, think, take your time, look at good sequels and ask what they did differently from bad sequels. One of the big reasons I see at work for people stopping series is that the books "got to be all the same," and of course, it's common knowledge that when Hollywood makes a sequel, it's not going to be as good.* Lack of audience interest can never be a good thing.
Bear in mind that this is merely the opinion of an unpublished, unsequelled writer who hasn't had time to read as widely as someone twice her age. I could be entirely wrong about all of this. I certainly don't have medical training, so I may have misdiagnosed the types, or missed a type entirely. But I've enough of an ego to say, "I think I'm right." I think that a lot of writers want to write sequels, for various reasons, and I think a lot of those times, those reasons don't exactly help the story they want to tell.
* With several exceptions, of course.