One of my standard - and fairly true - responses to the question as to how story ideas come to me is that story ideas only come to me for short stories. With longer fiction, it is a character (or characters) coming to visit, and I am then obliged to collaborate with him/her/it/them in creating the story.
Roger ZelaznyDid you ever look back at some moment in your past and have it suddenly grow so vivid that all the intervening years seemed brief, dreamlike, impersonalโthe motions of a May afternoon surrendered to routine?
Roger Zelazny