“Man’s best friend” is a common phrase about domestic dogs, referring to their millennia-long history of close relations, loyalty, and companionship with humans. The first recorded use of a related phrase is by Frederick the Great of Prussia.
Before the evolution of wolf into dog, it is posited that humans and wolves worked together hunting game. Wolves were the superior tracker but humans were the superior killer; thus wolves would lead humans to the prey and humans would leave some of the meat to the wolves. This working relationship eventually led to the evolution of dogs, though there is controversy as to the exact nature of that transition. Some say wolves evolved naturally into dogs, wherein the wolf that worked best with humans slowly began to assimilate and pass their domesticated genes down. Others say that humans took wolf cubs and raised them to be domesticated. Either way, humans and dogs formed a working relationship.
Previous to the 19th century, breeds of dogs (other than lap dogs) were largely functional. They served for activities such as hunting, watching, and guarding, language describing the dog often reflected these positions within society. According to the Oxford English Dictionary, “In the oldest proverbs and phrases dogs are rarely depicted as faithful or as man’s best friend, but as vicious, ravening, or watchful.” Beginning in the 18th century, multiplying in the 19th and flourishing in the 20th century, language and attitudes towards dogs began to shift. Possibly, this societal shift can be attributed to discovery of the rabies vaccine in 1869.