The history of the phrase "A dog is man's best friend" goes back to September 23, 1870 when lawyer George Graham Vest made his closing arguments in a courthouse in Warrensburg, Missouri.
The case involved a dog, Old Drum, who was shot by a farmer who saw the canine on his land. The farmer had been losing sheep to dogs and Old Drum was an unfortunate victim. George Vest won the case by convincing the jury that the best friend of man is his dog. The term has become a well known saying for dog lovers ever since.