Context: As I am writing my first paper, I cannot determine what, when and how I should cite scientific literature. It is because I don't know why I should cite (or not cite) other work.
My fist 'newbie' approach was "Every assertion you write should be proven by a relevant citation, so that other can believe you". Given that "I know nothing", no one would then take my word for it and thus I must prove that what I'm saying is true. However, this lead to overcitation (and, I must admit, over-generalization of results), and frustration since I must find a paper corroborating what I want to say for every sentence, or so
Then if I'm leaving this strict rule, I tend to think I am writing triviality, or affirming things I am not 100% sure. I also fear to unintentionally plagiarize, when I say something someone else already stated.
Question: In a scientific writing, why do we [have to/need to/should] cite?
Clarifying this might help answering corollary questions: what to cite? and when should we cite?
What to cite? Should I cite a paper (i.e. give credit) for ideas that are in its literature review, and that are totally not related to the core/added value of the paper? Should I cite every paper related to one field? Can I cite only not-that-much-cited source and not classic ones? etc.
When to cite? If I am writing an assertion that is general (e.g. "Polar bears have mostly white fur"), should I justify it?(!) Which is the criteria to determine when an assertion need to be proven by a citation, and when it is admited? When I define a key term for my paper, should I always rely on past definitions?
Disclaimer: Of course, I am aware of some obvious reasons for why we cite, such as giving credit and proving assertion, and I've read Arno's answer (who lists giving credit, for proof/evidence, and providing context as reasons) on a similar question. The originality of this question would be, to detail the link between the theory (why) and the practice (what, and when) of citing.
Furthermore, I am conscious that there are multiple questions in this post, especially in the bullet point list. However, these are just here for explanation purpose, and should of course be asked separately to get a specific and detailed answer.
* I know this should be done the other way (i.e. basing what I want to say on literature, and not looking for someone who could corroborate what I want to say)
You are telling me whatever you wrote is your pure thoughts and based on nothing else? If then, bravo! You don't need to cite and you shouldn't. However, if it is based on some other people contribution, the reviewer will easily reject your paper based on your title/abstract and not have any citation.
Think about it as a simple logical sentence. Assume that A, B, C are others people work, so you have:
A && B && C => True
Now you add your publication D which is based on A, B, and C, and you think your contribution is true:
A && BB && C && D => True
Then the reviewer finds out, yes it holds. So from now on your research community know about D; and if other people want to keep adding publications they need to cite D.
Confusion on Citations Is Curable By Reading More Publications
Yes we cannot just cite everything, however, it is always goes back to lack of reading. That is why you see academics need to keep reading all other papers if they want to keep publishing. So I would recommend you to read more and see what other people in your field are citing and contributing.
"Giants" Are Shifting
Same as a kid sees a red balloon and thinks it is the best invention, when you start your research, every little thing impresses you and you really do not have a solid foundation and understanding on who is the real giant in your field. This goes back to reading again. So, after reading more and more you will see who is the real "giant(s)" in your research topic; which then you need to cite for your work.