A good short-story writer has an instinct for sketching in just enough background to ground the specific story.
The only reason for being a professional writer is that you can't help it.
Chekhov - shall I be blunt? - is the greatest short story writer who ever lived.
My favorite short-story writer is John Cheever
You learn by writing short stories. Keep writing short stories. The money's in novels, but writing short stories keeps your writing lean and pointed.
To survive, you must tell stories.