The downward sloping demand curve is dictated by the law of demand. The law of demand states that all else the same (such as prices of other goods and income), the quantity demanded for a good declines with the price of the good. If we plot the demand curve with price on the horizontal axis and quantity demanded on the vertical axis, then it follows that as price travels along the horizontal axis to the right, the quantity demanded will move towards the origin along the vertical axis. This implies that the curve must be downward sloping.