Why does the aggregate demand has the downward slope ?

I found out on internet and I ll summarise it as 3 main explainations :

- Interest Rates :
In the economy, we have a fix amount of money. The price level representing for the amount of currency we have to spend to purchase something. If the price level is high, therefore we have to have a large amount of currency to purchase. And with low price level, the quantity of currency we have to purchase is less. Suppose that some of these money that we do not spend, will go to bank as a saving. It will lead to the rising in bank loans and therefore it will drive down the interest rate.
With lower interest rates, firms are easier to borrow and therefore the investment will increase, which leads to the increase in aggregate demand ( because Investment is one of factors determine the aggregate demand).

- Exchange rates :
When the price level falls, the interest rates is driven down and therefore it more likely to decrease the exchange rates. Lowering exchange rate can increase the net export because the price of export become cheaper. Price level falls, exchange rates fall, increase in net exports which contribute to aggregate demand >> therefore AD increase.

- Wealth :
The lower price level is, the more you can afford to buy >> therefore it increases the consumption and leads to the increase in aggregate demand.