In mainstream fiction [...] we cannot succumb to the whim of the story. We can’t decide that the reason the barista won’t date the main character ISN’T because she’s had a horrible breakup and is slowly learning to trust again (leading to series of bad lovers because she feels more comfortable when she KNOWS she can’t trust) but rather because there is a dragon’s ghost within her, and love and lust can only be fulfilled if that dragon is defeated by creating a mythical cup of cappuccino that transports the main character to a fantasy world, and also goes quite well with bagels or croissants.
--Paul Tobin explains why he enjoys writing genre stories better than slice-of-life stories. As you can see from the quote, it has to do with the joy of creative freedom, but he also balances that in the article with the need for rules, even in a fantastical setting.
It's a mix that's tough to get right, but as a reader it's the best thing in the world when a story can not only connect me to other people through its characters and our shared humanity, but can also throw in some vampires, pirates, and creatures from space just to keep it interesting.