Introduction: The demand curve is a graphical representation of the relationship between the price of a commodity and the quantity demanded by consumers. It is a fundamental concept in economics that helps to explain various phenomena in the market. The demand curve always slopes downwards and this is due to various reasons.

Law of Diminishing Marginal Utility: One of the main reasons for the downward slope of the demand curve is the law of diminishing marginal utility. According to this law, as a consumer consumes more and more units of a commodity, the marginal utility or satisfaction derived from each additional unit decreases. As a result, the consumer is willing to pay a lower price for each additional unit of the commodity, leading to a decrease in the quantity demanded at higher prices.

Income Effect: Another reason for the downward slope of the demand curve is the income effect. As the price of a commodity decreases, consumers' purchasing power increases, allowing them to buy more of the commodity. This results in an increase in the quantity demanded at lower prices.

Substitution Effect: The substitution effect is another factor that contributes to the downward slope of the demand curve. When the price of a commodity decreases, it becomes relatively cheaper compared to other similar commodities. This encourages consumers to switch from other commodities to the cheaper commodity, resulting in an increase in the quantity demanded at lower prices.

Market Saturation: Market saturation is another reason for the downward slope of the demand curve. As the market becomes saturated with a particular commodity, consumers may start to lose interest in it. This can lead to a decrease in the quantity demanded, even at lower prices.

Conclusion: In conclusion, the demand curve slopes downwards due to various reasons, including the law of diminishing marginal utility, income effect, substitution effect, and market saturation. Understanding the factors that contribute to the downward slope of the demand curve is crucial for businesses and policymakers to make informed decisions about pricing and production.

The demand curve slopes downward because there exists inverse relationship between price and demand.Now the question arises why there is negative relationship between price and demand.So, it happens because of following reasons:
1. Law of Diminishing Marginal Utility
2. Income Effect
3. Substitution Effect
4. Size of Consumer Group
5. Several Uses of a Commodity