Lower classed citizens of the BNW are conditioned to hate books and flowers because a love of nature does not consume resoures, does nothing to increase the economy, and is a pastime that does not contribute to social stability or the good of the BNW. Books are also a part of the conditioning because through books one can gain knowledge, get more intelligent, and become unhappy with one's station or caste in the BNW. An appreciation of nature and beauty is not essential to the economy, survival, or each of the castes.
It was in chapter two of the novel that conditioning at the hatchery is practically explained to the students by the director. The director wanted to show the students exactly what happened in the infant nurseries. The director asked the nurses to place books and flowers where the infants could see. He then asked for the delta children to be brought and once exposed to the sight of the books the children crawled towards the items in excitement. As they reached close enough to touch the items the director asked the nurse to press a button that created an irritating noise and another button that sent electrical shocks to the children. All this was done to condition the children against books and flowers, however one of the students pointed to the director that although the issue about books was obvious the point of conditioning against flowers remained elusive. It is at this point that the director points out that if the children of the lower castes were left to love flowers no work would be done at the factories and product purchases will drop leading to a failure in the economy.
In summary, the conditioning of the Deltas against flowers was with regards to the economy. Their social class required them to be both the workers and the consumers and an infatuation with flowers would interfere with these activities economically.