I spend a long time messing around with a story in my head before committing to it. Changing things, adding things, discarding things, all things small and big. The moment that makes me go "this is a story worth telling" is when the story has a central theme that speaks to me. After that, everything starts clicking into place. I still have a ton of work to do, patching up the plot holes, doing worldbuilding, etc... but it all becomes a smooth process for the most part.
It's kind of like what @digitalvalium1 said:
Funny thing is, I can't seem to pick a theme I like and build a story around that. For one thing, the result tends to look forced. For another, often I don't KNOW what is the message that needs to get out of my heart the most. I won't know it until I see it in my work. But when I do see it, I'll recognize it immediately, and everything will make sense.
I'm also deeply in love with my characters. I don't believe in my ability to tell their story, but I believe in THEM. In my heart, there is no doubt that the story I'm trying to tell is beautiful -- even if my attempt at telling it isn't successful. It's kind of the writing equivalent of seeing something (a rose, a sunset, a character design) so beautiful that it compels you to try drawing it, even if you know you won't be able to do it justice.