Therefore, most goods have demand curves with negative slopes. Individual demand curves slope downward because of the law of diminishing marginal utility. Market demand curves are the sum of individual demands, so market demands should also have a negative slope. Market demands may slope downward even if individual demands do not, because most consumers are different. Think of the market for beef.
People differ with respect to their tastes for beef.
If the price of beef rises, avid beef eaters may continue to buy beef at high prices, while lesser beef enthusiasts may drop out of the market entirely. It is the disappearance of those customers that accounts for the negative slope of market demand curves versus individual demand curves. Thus, as the price falls, Joe will buy more and Leah will enter the market, insuring that quantity demanded increases as price falls. If the price of generic aspirin is less than Bayer, people may buy less of the generic brand. If the price of a Rolls Royce fell, people may buy fewer of them.