That's one key question for someone seeking to develop what Lajos Egri calls "the premise." It's that undergirding message that flows through your work. It's not something you stand on a soapbox and preach to your audience, but it is something that reflects the science of human nature.
Why is it essential? For one, because people naturally gravitate toward works that say something. Maybe it's simple, like "Good triumphs over evil," but at least it's something. The premise allows you to set characters in motion and watch them play out exactly what it is you're trying to get across.
So, how do you create one? Egri gives us a simple formula: character + conflict + resolution = the premise (all right, so that's my interpretation of his work). Start with something that you're passionate about. I like to slog through a huge list of virtues or emotions, things like: diligence, faithfulness, honesty, ruthlessness, jealousy, etc. Select the most meaningful one you can.
Now consider that virtue (or emotion). What result would that have if played out in a certain scenario? Let's take jealousy. Okay. What's the result of jealousy? It blindly destroys love! It festers until it destorys the jealous person and sometimes even his or her lover...
Guess what, you've just defined the premise of Othello: "Jealousy destroys itself and the object of its desire."
I think Mr. Egri is onto something, don't you?