The idea of staying in good humor or humoring those around us has an ancient lineage. It actually traces back to Greece and the humoral theory of medicine.
"To begin at the beginning: the elements from which the world is made are air, fire, water, and earth; the seasons from which the year is composed are spring, summer, winter and autumn; the humours from which animals and humans are composed are yellow bile, blood, phlegm, and black bile." --Galen
For centuries, the idea that an excess of phlegm or of yellow bile could cause illness was an accepted medical diagnosis. The four humours--yellow and black bile, phlegm, and blood--circulated throughout the body and an in balance in one or more were believed to be the cause of illness. The theory began in the 5th century BCE with work attributed to Greek physician Hippocrates (though it was his son-in-law and disciple Polybus who wrote the first treatise that clearly explained the whole idea of the humours) and continued with Roman doctor Galen, who adopted the theory in the 2nd century CE. For the next two thousand years (give or take some disruptions and such like the sacking of Rome), humoral theory explained most things about a person's character, medical history, taste, appearance, and behavior.
Why? What was so compelling about this theory?
Well, for one, it seemed to unify passions and cognition, physiology and psychology, and the individual and his/her environment. Various parts of the body and the environment caused disease and stirred various emotions and passions. The humours also made sense to many cultures of people who based their earliest stories of creation on four elements: air, fire, water, and earth. Each of the four humours was tied to one of these elements so it seemed a natural extension of what people already knew about the world. Our human need to understand what we are made of, where we came from, and how we work often causes us to resort to structures and traditions that match our intuitions. The theory offered a potent image of substances, particles, or currents traveling through the body from the limbs to the organs to the brain and heart and back. It seemed to explain how the sight of an attractive person could trigger desire, induce a rush of blood in the veins, and increase the heartbeat.
The fall of Rome wasn't the end of the humoral theory, though. It was more of a shift--to the east to Islam where the knowledge of the Greeks and Romans was saved and expanded upon, and to the abbeys were monks preserved ancient texts. (sidenote: In researching my book on apples, I discovered how much of ancient knowledge was preserved in the Islamic and Christian monastic traditions on apple orcharding and fruit growing in general. So it wasn't too surprising to find that medical knowledge, too, lived on in these same places.)
And so the theory lived on and on, taking on various forms to fit the times but always coming back to the idea of balance and imbalance in the body as the source of illness. The theory only really died with the discovery of the existence of germs in the late 19th century. Not everyone bough the germ theory right away, however, so books based on humoral theory continued into the early 20th century.
Humours now remain mostly familiar in our expressions about keeping balanced and experiencing something with ill-humor. In French, the word for mood is humeur. Many Asian medical traditions are also humoral, based on the idea of energy flows, mind-body connections, and balances between hot and cold, moist and dry. So even if the theory is no longer used to describe disease (at least in the West), the idea of humours still serve as useful and suggestive images in our culture.