In this novel, humankind must make decisions after surviving a disastrous "Nine Years' War" that caused great suffering and was followed by an economic collapse (not unlike World War I and the Great Depression.) The society that emerges from the wreckage wants peace, quiet, and stability at all costs. As the Controller says to the Savage:
What's the point of truth or beauty or knowledge when the anthrax bombs are popping all around you?
Conditioning, along with liberal use of a drug called soma, becomes the primary way this new society achieves peace, security and superficial happiness. People are genetically designed in a test tube to fit the characteristics of a certain caste and then conditioned from birth to be satisfied with the life that has been predetermined for them. They are not raised in families (parenthood and childbirth are considered obscene) but in dormitories, where they are conditioned every night by tapes that play while they sleep to want what the larger society has decided they should desire and to despise the attributes of other castes. By day, they are conditioned through pleasure and pain to embrace a shallow materialism, while any problems are smoothed away with soma. As far as the Controller is concerned, truth, beauty, poetry, love and intensely felt emotions are a small price to pay for a world without suffering and disorder.
The world controller conditions the embryos and the young children of Brave New World because conditioning certain castes to fit into their designated roles, the society is stable. Conditioning is a valuable tool in the name of social stability. Conditioning occurs as early as the embryonic stage as the embryos travel down a conveyor belt. Alphas are given more oxygen so that they develop into intellectual leaders, and the Epsilons are deprived of oxygen or alcohol is given to make the individual less intellectual and more of a hearty worker. Hypnopaedia is another valuable tool for social stability because it keeps all of the classes in line and behaving correctly in their social roles. When Lenina repeats her hypnopaedic sayings almost mindlessly, she's demonstrating the almost unconscious and very strong role of conditioning.