How to choose keywords for papers?
Since keywords are meant to be used as search indices for certain topics, allowing authors to generate their own keywords makes little sense. For all journals I know, keywords have to be selected from a list published by the journal. Some journals have adopted common keyword lists (for example most journals in astrophysics use this list).

Keywords in academia play almost exactly the same role as tags on SE sites (except that author cannot invent new keywords). In particular, their selection by the authors follows the same principle: find the most suitable set of keywords for the given study, such that a keyword search brings up the most relevant articles.

Keywords are perhaps not as important now as they used to be since electronic word searches on title, abstract or the whole article.