Why indifferent curve must be downward sloping?
The normal downward sloping indifference curve assumes that monotonicity holds. This means that an increase in the consumption of one good increases overall utility, and of course the opposite is true, a decrease in the consumption of one good decreases overall utility If this assumption holds, the indifference curves will be downward sloping. Using an example, if the two goods are chocolate and pizza, to maintain the same level of utility from a decrease in the consumption of chocolate (which decreases overall utility), your consumption of pizza must increase (to increase your utility back to the original level), creating a downward sloping indifference curve. It’s probably easiest if you think about this graphically. However, not all indifference curves must be downward sloping. There are special cases where monotonicity does not hold, such as 1. One of the goods is a bad. If a good is a bad (an increase in consumption decreases utility), the indifference curve will be upward sloping
