the writer feels they have nothing unique to say.
After all aren't there only so many different ways to rewrite the same old cliched plot of boy meets girl, girl meets other boy, girls gets ill/dies/has to make a life defining choice between the two men/wolves/demons in her life.
So why bother to write at all if you haven't got an original idea?
And if we ignore the obvious logical question of how do you know if your story is original, after all there are so many books published every day, how would you know if your book has an original angle unless you have read them all.
The real concern is not the concept of plagiarism where you deliberately try to pass other peoples work as your own but the fact that sometimes you can do this without being conscious of it.
Lets get down to details. My first published book (the other four are still in editing stages) is about vampires and cloning. It's genre fiction, therefore there will be a lot of elements I have chosen to use which are standard to the genre, i.e. vampires drink blood, and there are a lot which I have discarded because they do not fit with my own view of what I want vampires to represent, for example, combusting in sunlight. It's so cliched dear! I want my vampires strutting no matter the weather.
To me vampires are people who rather than luxuriate in their immortality would probably fail to enjoy it; just like normal people they would fail to seize their opportunities and would ultimately become frustrated and bored.
The reason I wanted to write about vampires is I am a mad fan of Buffy the Vampire Slayer, Sookie Stackhouse, Twilight, and Laurell K Hamilton; though deep down, my heart will always belong to the Buffster.
When I read a book I become involved with the characters and engrossed in their world. So even when I have left that world, part of that world comes with me.
If I then sit down to generate my own story then surly it would be almost impossible to do it without a hint of Buffy and a pinch of Sookie, and lets face it in genre fiction that's what is expected.
If you want to read about vampires then you want some of the elements that make the book part of that genre, vampires drink blood and are sexy and rich, because that's part of why you like it, but you also want something slightly different. That's why when you scan for a new book there is a frisson of excitement; you want something old as well as something new, in a perfect combination.
So from a writers point of view its simple, trust your subconscious to weave your past readings with your future dreamings and simply write.