“Be a sadist.
No matter how sweet and innocent your leading characters, make awful things happen to them—in order that the reader may see what they are made of.”
Kurt is right. The more dire the circumstances, the better the story. You want readers to root for your hero and heroine and in order to accomplish that they need to face their worst fears. They need to be put in terrible circumstances.
When plotting your story, or writing from the seat of your pants, ask yourself “What’s the worst that can happen?” and then make your character endure it.