The demand curve represents the relationship between the price of a product or service and the quantity demanded by consumers. It is a downward-sloping curve, meaning that as the price of a good increases, the quantity demanded by consumers decreases, and vice versa. This relationship can be explained by several key factors:

1. Law of diminishing marginal utility:
The law of diminishing marginal utility states that as individuals consume more units of a good, the additional satisfaction or utility derived from each additional unit decreases. In other words, the more of a good a person has, the less valuable each additional unit becomes. As a result, consumers are willing to pay a higher price for the first unit of a good and are generally willing to pay less for additional units. This leads to a downward-sloping demand curve.

2. Income effect:
When the price of a good decreases, consumers are able to purchase more of it with their given income. This increase in purchasing power leads to an increase in the quantity demanded. Conversely, when the price of a good increases, consumers may not be able to afford the same quantity with their income, resulting in a decrease in quantity demanded. This income effect contributes to the downward slope of the demand curve.

3. Substitution effect:
The substitution effect occurs when consumers switch from one good to another as the price of one good changes relative to another. When the price of a good increases, consumers may seek out cheaper substitute goods, leading to a decrease in the quantity demanded of the more expensive good. Conversely, when the price of a good decreases, consumers may switch from substitute goods to the now relatively cheaper good, resulting in an increase in quantity demanded. This substitution effect also contributes to the downward slope of the demand curve.

4. Law of demand:
The law of demand states that there is an inverse relationship between price and quantity demanded, holding all other factors constant. This means that as the price of a good increases, consumers are less willing and able to buy it. On the other hand, as the price decreases, consumers are more willing and able to buy it. The law of demand is a fundamental principle in economics and provides a theoretical basis for the downward-sloping demand curve.

In summary, the downward slope of the demand curve can be explained by the law of diminishing marginal utility, the income effect, the substitution effect, and the law of demand. These factors collectively influence consumers' behavior and their willingness to purchase a good at different price levels.

It is because when other factors remain constant (expectations , price of related goods etc)there is a inverse relationship between price and quantity demanded of a commodity means when price of a commodity increases quantity demanded decreases and vice versa. For example when price of commodity increases from 10 to 20rs. It's quantity demanded is decrease from 20 to 10 and vice versa.