The demand curve slopes downward because a rational consumer always demands more of a commodity when its price falls and less of a commodity when its price rises. There are three basic reasons for that:

• Substitution effect-> When the price of the commodity falls, it becomes cheaper in terms of the other goods. Therefore, the consumer would like to shift his/her demand from the other goods to the given good.
• Income effect-> When the price of the commodity falls, consumer's real income rises that is he can now purchase more of the commodity with the same income.
• Law of diminishing marginal utility-> It states that the satisfaction derived from the commodity will diminish from every successive unit. So the consumer would be willing to pay less and less for each successive unit.