In order to explain the negative slope of AD curve, we cannot use the logic that we used to explain the negative slope of individual demand curve. Since money demand increases when prices increase, there is a negative relationship between prices and aggregate demand. In order to work of this process, while money supply is constant, interest rate must increase to ensure the money market equilibrium. Therefore, to understand what aggregate demand curve tells us, we should understand the interaction between real and financial sectors of the economy.