THEORY OF CONSUMER BEHAVIOR:

It is generally observed that market aggregate demand curve for a commodity is downward sloping, given other things. Our problem is to investigate economic rationality behind this for a commodity of all individual consumers. The market demand basically depends on the characteristics of demand for a commodity by individual consumers, and the demand for a commodity of an individual consumer depends upon the behaviour of the consumer. Clearly, to  investigate economic rationality behind the law of demand, we shall start with the analysis of consumer behaviour.  

There are different approaches to analyse the consumer behaviour. But in all approaches, it is assumed that the consumer is rational. This means that the consumer's objective is to maximise her utility by choosing one commodity bundle from among all the commodity bundles (money income and the prices of the commodities are given to the consumer).