I've read several blog posts about where a certain story came from and how it evolved from an idea into a full-fledged novel. I can relate, though my ideas aren't nearly as lofty-or coherent-as those posts I've read.
A Venetian Affair: A True Tale of Forbidden Love in the 18th Century by Andrea Di Robilant. If you haven't read it, you should-it's the true story told from family history and discovered (and encrypted!) letters between the lovers.
I loved the book and became obsessed with Venice from then on. When I wrote my first (contracted I should mention) short, I wanted to do something about the era, to capture the flavor of the city which was so very different than any other in Europe.
I don't know why I chose to make it a threesome, other than that was what the anthology call was for-threesomes. I find the merging of three lives fascinating, to see how they live and survive. it's hard enough with two people, adding in a third there's always the risk someone might feel left out or ignored.
Makes it more interesting, I think. :) Don't you?