Why does the aggregate demand curve slope down?

Aggregate demand (AD) is a curve showing the total amount of goods and services (real GDP) that will be purchased at different price levels.
As in the AD,AS diagram u always can see that AD curve slopes down.It has a negative slope.Why???

Three explanations for the negative slope are:
1. Interest rate effect
2. Real balance effect
3. Foreign purchases effect

• Interest rate:

Well, as price level rises so therefore the money supply will fall.Bcause real money supply falls,this leads to an increase in the interest rate.As interest rate rises, investment spending falls because of the increased cost of borrowing to finance investment. As investment falls, aggregate expenditure (AE) falls, in turn leading to a fall in real GDP (or Y).

• Real blance effect:

Real balance effect (also known as real wealth effect or real money balances effect):
It is proved through consumption and expenditure. As price level rises, purchasing power of money balances falls. As our real wealth falls, we will cut back on our consumption spending,which means consumption will decrease.Therefore,this will leads to a decrease in Real GDP in the economy.

• Foreign purchases effect:
  It is applicable only to open economies with foreign trade being introduced into our explanation. With an increase in domestic price level relative to those of trading partners, domestic consumers will spend less on domestically produced goods and services and more on imports (M). Also, foreigners will buy less of our exports (X). The increase in M and decrease in X (that is, a decrease in net exports) means a decrease in real GDP.

To explain why AD slopes down,we can prove by an increase in the price level leads to a decrease in the Real GDP,therefore,AD slopes down to show that correlation.