Why do demand for substitutes and demand for the original goods tend to move in opposite directions?

Demand for substitutes and demand for original goods tend to move in opposite directions due to the relationship of consumer preferences. When the price of an original good rises, consumers are likely to seek cheaper alternatives, increasing demand for substitutes. Conversely, if the price of the original good falls, it becomes more attractive, leading to a decrease in demand for its substitutes. This inverse relationship reflects consumers' tendency to switch between goods based on price changes and perceived value.