Reasons for downward sloping demand curve:

• Law of Diminishing Marginal Utility: According to this law, as consumption of a commodity increases, marginal utility of each successive unit goes on diminishing to a consumer. Accordingly, for every additional unit to be purchased, the consumer is willing to pay less and less price. 

• Income Effect: Income effect refers to change in quantity demanded when real income of the buyer changes owing to change in price of the commodity. With a fall in price, real income increases. Accordingly, demand for the commodity expands.

• Substitution Effect: Substitution effect refers to substitution of one commodity for the other when it becomes relatively cheaper. Thus, when own price of commodity-X falls, it becomes cheaper in relation to commodity-Y. Accordingly, X is substituted for Y. Tea and coffee are substitutes. With a fall in the price of tea, it is substituted in place of coffee. It is expansion of demand due to the substitution effect. 

• Size of Consumer Group: When price of a commodity falls, many more buyers can afford to buy it. Accordingly, demand expands. 

• Different Uses: A good may have several uses. Milk, for example, is used for making curd, cheese and butter. If the price of milk reduces it will be put to different uses. Accordingly, the demand for milk expands.