... Well, they have to revolve around an issue.
Why is the story sad? Did someone die? Does someone have cancer? Is the main character just in a shitty situation?
From this basic idea, you build your story. Who was the person that died, and what did they mean to the main character? Who has the Cancer, and how bad is it? What circumstances make the Main Character's life so crap?
By that point, a story should be forming in your noggin. Just grow it and see where it takes you.