It all comes down to blood pressure. When you stand up too quickly, gravity makes the blood in your head rush to the lower part of your body, thus causing blood pressure in your upper body (especially head) to drop, therefore causing dizziness or lightheadedness. You might be wondering, "well then why doesn't this happen every time we stand up?" Usually, blood vessels in your upper body tighten when you stand to aid in stabilizing your blood pressure (and if you stand up too fast or have a medical condition such as low blood pressure, you're more likely to feel dizzy because of poor blood circulation to your head). Also, this dizzy feeling is called orthostatic hypotension, and you can read more about when to see a doctor, causes, risk factors, and complications here.