Reasons for the Law of Demand
The law of demand states that as the price of a good or service decreases, the quantity demanded of that good or service increases, and vice versa. There are two main reasons for the law of demand:

Substitution Effect
The substitution effect is one of the key reasons behind the law of demand. When the price of a good decreases, consumers are more likely to switch from relatively more expensive goods to the now cheaper good. This is because consumers tend to seek the best value for their money. For example, if the price of coffee decreases, consumers may choose to buy more coffee and less tea because coffee now offers better value for their money.

Income Effect
The income effect is another important reason for the law of demand. When the price of a good decreases, consumers effectively have more purchasing power. This increase in purchasing power allows consumers to buy more of the now cheaper good, leading to an increase in quantity demanded. Conversely, if the price of a good increases, consumers may feel like they have less disposable income and therefore buy less of that good. For example, if the price of smartphones increases, consumers may choose to buy fewer smartphones and opt for more affordable alternatives.
In conclusion, the law of demand is driven by the substitution effect and the income effect. These two reasons explain why consumers tend to buy more of a good when its price decreases and less of it when its price increases.