Does considering someone's work regarding a piece of literature constitute plagiarism if you knowingly derive/spark ideas from it and don't cite it? Is reviewing with/considering someone's summary/analysis of the events of a piece of literature plagiarism if you write a piece that has ideas which may have sparked from considering that piece?

Put less abstractly, for example, if some scholar (call him: John) were to be writing a piece on Catcher in the Rye, and made a web-search for "sad moments" in the book Catcher in the Rye and came across what some author expressed as sad moments (call it: Sad CITR Moments) in the same book would it be(?) plagiarism if:


A. John reads Sad CITR Moments and it reminds them of something he too recalls being sad, and then goes on to write about it without citing the paper.
B. Same as A, but John forgot one detail which makes it more/less sad, hence he goes on to write more/less strongly about its emotionality.
C. John didn't consider something before, but the argument put forth in Sad CITR Moments on the matter convinces/de-convinces him and he then goes on to write about it in the affirmative/negative position.


I'm sure one could think up more cases (and do share if you do) but I'd hope the basic idea to be clear at this point.
Why not cite things clearly if you know where your own idea 'derives/sparks from'? Laziness? Shyness? 

Citing somebody else's work won't diminish the quality or significance of your own contribution, on the contrary. 

And readers will be able to understand your foundations and line of thought better as a result.

And ultimately it saves you a lot of headaches, as you don't have to worry if you should - have - cite(d) other people's work that stimulated your thought.