Aggregate demand curve is nothing but the sum total of all the
demands In the economy in terms of purchases made by the
government, consumptions, investments made etc at each price level
at any particular period of time.
It slopes downward as the price level and the output are
inversely related to each other.
As output decreases, price level increases and vice versa.
Also, as price level increases, national income also drops down and
vice versa.
It slopes downward also because demand and price level are
inversely related to each other.
As the price level decreases, goods become competitive due to
higher demand leading to greater exports.
Also, as the price level decreases, income of the consumers
rises up leading to increased spending over goods.
For above all the reasons aggregate demand curve slopes
downward.