Demand curves usually slope downward because of the income and substitution effects, and because of the law of diminishing marginal utility.

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The income effect suggests that as the price of a good decreases, the consumer's purchasing power increases and they are able to afford more of that good or other goods, leading to an increase in quantity demanded. The substitution effect suggests that consumers will switch from a more expensive good to a cheaper one, leading to an increase in quantity demanded of the cheaper good. Both of these effects lead to a downward-sloping demand curve. The law of diminishing marginal utility suggests that as a consumer consumes more of a good, the additional satisfaction they derive from each additional unit of the good decreases, leading to a decrease in willingness to pay and a downward-sloping demand curve.

Demand Curves Usually Slope Downward Because of the Income and Substitution