Law of Demand:

There is an inverse relationship between relative price and the quantity demanded, other things equal (ceteris paribus). In other words, a downward sloping demand curve.

Reasons for the Law of Demand:

Substitution Effect: The tendency for people to substitute in favor of cheaper goods and away from more expensive ones is the substitution effect.

Real-Income Effect: The change in peoples purchasing power when the price of one of the goods they buy changes is the real-income effect.