Your intentions are what set the wheels of creation into motion. What you intend and give attention to (in other words, think about), you create.
So why do people wind up with so many unintended consequences in their lives? Because they don't deliberately set their intention and if they do, they don't follow it up with positive thought about what they desire. If your intention, for example, is good health, but the focus of your thoughts (and words you speak) is on how poorly you feel, then your focus is on poor health, and your thoughts and words have overridden your intention. If, however, even when you are feeling poorly, you set your intention for good health, find things about your health for which you can express joy (my body breathes beautifully, my legs support me, my body uses the nutrients I give it efficiently, etc.) and then change the story you tell about your health to one that is positive, joyful, and supportive of good health, then your body will respond.