When you first come up with an idea for a story, you don’t have to think too hard about what it is you like about the story. Something catches your interest. One idea follows another and you’re off and running.
Later, when you’re deep in the belly of the beast, maybe stuck in the middle of the first draft, or struggling with the umpteenth rewrite, the very point of writing as a use of your time comes into question. You liked the idea as an idea, but this sprawling mess in front of you doesn’t seem to be that thing at all.
Why are you even bothering? Who is going to read this? Aren’t there already a thousand stories like this? What’s on TV?
You have to be able to hold onto the thing that made you want to write this particular story. When the going gets tough (and it will) you need that thing to get you through. But first you need to work out what thing is.