This occurred to me, more or less irrelevantly, while I was reading a story in Asimov's recently:
Fiction is often set at the time in the characters' lives when something new happens. Someone or other once said that most stories essentially begin with "And then, one day, . . ." (I think we had an argument about this at one of the SH workshops, so I should note that I do not in any way mean to imply that all stories are like this, or that stories should be or have to be like this. But I think it's pretty common, especially in genre stories.)
And sometimes I get the feeling that this is the first time anything particularly important or unusual or exciting has ever happened in those characters' lives.
But I think one of the things I love about some stories is when I get the sense that this isn't the first important or unusual or exciting thing that's ever happened to these people. Especially when I get the sense that things have happened to them in the past that are so important or unusual or exciting that the author could've written another story (or book) about those past things.