To decide which stories are truly worth telling, I ask myself a series of questions.
1) Does this character start out at some kind of disadvantage? Struggle makes an interesting story, and readers love underdogs.
2) Does the character experience a significant change or growth or some sort, and will the story of that change hold a reader's attention? There has to be an interruption of the norm to create a story, and the events of that interruption have to be significant enough to want to write about.
3) Will the resolution of the story send some kind of message? I'm never worried about whether readers will "like" the endings of my books because I love leaving readers with an unexpected ending. However, I do want to make sure I leave them with a subtle message to contemplate.
4) (and this is the most important one) Does the idea of writing this story make my heart race? Am I excited about writing this story? Do I look forward to picking up my pen and pulling out my notebook to work on it? I only write stories I'm passionate about, so it has to move me for me to pick up my pen and put it to paper.