John B. Watson's theory of behavior explains all human actions as responses to stimuli. His practice of the science of behaviorism, which studies what people do and makes predictions as a result of those observations, showed him that human conditioning comes as a result of inner reactions to stimuli.
Environmental conditioning comes from a variety of sources, and the earliest tend to come from input from parents and other caregivers. One of the most common words that toddlers hear is "no" as they start experimenting with actions and words, learning from the reaction which are and are not appropriate. Walking up to a stove with the intent of yanking on the metal handle that extends from the front usually leads to a loud, negative response from the adult in the room. If not, the boiling water that is inside the pot connected to that handle provides another powerful environmental stimulus, even more painful than the spanking that the adult may have administered.
Over time, people learn to act as a result of this behavioral conditioning, according to Watson. With age come more and more sophisticated decisions with resulting responses. As consequences become more and more lasting, those who have successfully learned lessons from conditioning experience higher levels of success.