Why is the demand curve downward sloping?

The demand curve slopes downward because of an inverse relationship between price and quantity demanded, as stated by the Law of Demand. This downward slope is explained by the income effect, where lower prices increase real income, allowing more purchases; the substitution effect, where lower-priced goods replace more expensive alternatives; and the principle of diminishing marginal utility, where additional units of a good provide less satisfaction, making consumers willing to buy more only at lower prices.