Reasons for an aggregate demand curve sloping downwards
1) Pigou's wealth effect - drop in the price level induces consumers to spend more, thereby increasing the aggregate demand.
2) Keynes's interest-rate effect - a drop in the price level decreases the interest rate, which increases the demand for investment and thereby increases aggregate demand.
3) Mundell-Fleming's exchange-rate effect. - price level drops, interest rates fall, domestic investment in foreign countries increases, the real exchange rate depreciates, net exports increases, and aggregate demand increases.