Why does marginal rate of substitution change along the indifference curve?

The marginal rate of substitution (MRS) changes along the indifference curve because it reflects the rate at which a consumer is willing to give up one good for another while maintaining the same level of utility. As a consumer substitutes one good for another, the quantity of the first good decreases while the quantity of the second good increases, leading to diminishing marginal utility. This diminishing utility means that the willingness to trade off one good for another decreases, resulting in a changing MRS along the curve. Thus, the shape of the indifference curve illustrates this variability in trade-offs between goods.