The Hand of Fate should not write stories.
Coincidence. It happens all the time, and never more than in stories. Many narratives depend on coincidence, but is it a good or bad device? Like most tools, if used well it can make a story move neatly towards the conclusion, but if abused it reeks of lazy writing.
A lot of mysteries and thrillers will include coincidence as a way of bringing a crime to light (but not usually to reveal the culprit). Steven King masterfully winds coincidence into his horror stories to give them a dark fateful theme. So how should you use coincidence?
Narratives are small worlds or micro-communities. We don't want a cast of thousands in our story, so we zoom into a portion of life. This makes coincidences more likely to take place. Here's some tips I picked up from my recent writing course with Sydney Writers Centre.
Bring it in early. Have your coincidence set things rolling. Your protagonist has a fight with her boss when she's fired, later that day, while walking her dog, she drops her mobile phone. Not a good day for her. She starts searching the bushes in the nearby park for said phone, and what does she find? The dead body of her ex-boss. Coincidence. Now your protagonist is a suspect and has good reason to try to solve the crime herself. See how coincidence has moved your plot along?
Coincidence shouldn't be used to solve crimes though, we want to think our amateur detective is using their wits, not luck. And please don't end your story with coincidence - most readers hate that! (including me) Okay, some like it, and some fateful endings have been successful.
Ending with a coincidence, or 'by the hand of God', has a name: Deux ex Machina. This is where a plot problem heads to such a point it seems unlikely to be solved, and then, by the hand of God, all is resolved by fate. 'The Postman Always Rings Twice', is an example where it seems the two lovers will get away with murder, until fate steps in. Although a successful film, this ending always makes me feel robbed of a propper resolution. Having said that, I did like the ending to the first 'Sherlock Holmes' movie, but at least Sherlock used his wits to solve the crime first.