"For obvious reasons", "for good reason", please explain.
"For good reason" and "with good reason" are idiomatic using the singular "reason". I'm not sure there is any simple explanation, because other idiomatic sayings use the plural "reasons", for example:
- for unknown reasons
- for reasons best known to oneself
- for reasons unknown
You can say "for a good reason", and "for an obvious reason* if you want to highlight that there is just one citeable reason.