Sometimes you know you're done with a series because it brings you more negative than positive. You're bored, you don't care, you actively disliked the last book. That's when it's easy. You can feel the reasons why you should stop wasting your time and money.
But other times, it's less straightforward.
Other times it's like realizing a relationship isn't working. It's not bad, but it isn't working. You're not really hurting one another, but you're also not doing anything for one another either. And to bring it back to books, it can be harder in those cases, because you still enjoy reading it on some level. But you're not looking forward to it. It's like food that you eat because it's there, not because you actually want it. You might still like the taste, but there are better reasons to eat and better things to eat.
So many metaphors!
Long way of saying, this is where I left off collecting Hayate the Combat Butler. Book 17 gives us a lengthy backstory about Hayate and a young woman he was a butler for back in elementary school. The girl lived in a mysterious castle, had many impressive skills, seemed to know some magic or at least have some magic items. The book left off in the middle of this backstory, and at the end I found myself mildly curious about what would happen next. This was huge backstory stuff and I just... eh.
That being said, it is a nice volume - the story is interesting and cute, if a little weird (there's always been magic in the story, but usually not played quite this straight). It's not a bad volume, there's nothingw rong, I'd just gradually lost passion for reading this, and this was the point I finally realized it.