Why is an indifference curve negatively sloped? Explain.

An indifference curve always slopes downward from left to right, i.e. it has a negative slope. This is so because if a consumer wants to have more units of one commodity; he will have to reduce the number of units of the other commodity, due to his limited income. However all combinations of two commodities plotted on an indifference curve gives the same level of satisfaction and the cost of each combination is equal to the income of the consumer.