The demand curve is generally downward slopping because it depicts the relationship between price and quantity demanded. On the vertical axis is price and on the horizontal axis is quantity. This means that generally what happens is that when price of the product increases, lesser people will be able to buy it and hence the demand will decrease. With an increase in price, the quantity demanded decreases, which means that there is a negative relationship between the two variables. Curves are always downward sloping or inverted for variables that are inversely related to each other. An inverse relationship means that when the increase in one variable (price) causes a decrease in the other variable (quantity).
Demand curve slopes negatively because it slopes downwards from left to right.
Demand curve is downward or negatively sloping because increase in price will decrease the price of that commodity. So there is an inverse relation between the demand and price of the commodity which sl0pes negatively on the demand and price graph.