The phrase initiated in the 16th century when dogs would guard homes and small communities, were fed scraps, slept outside and had short lives. so it meant life wasn't good, whereas today dogs are well fed, groomed, pampered, sleep inside and live longer, so it now means a good life.
A Dog's Life is a 1918 American short silent film written, produced and directed by Charlie Chaplin. Chaplin seems to have found the combination of past and present definition in the plot to the movie. This from wikipedia:
"Charlie is jobless and has few prospects for employment. He tries to steal food from a lunch cart and is nearly caught by a police officer, avoiding arrest by doing some fancy rolling back and forth under a fence. Later, Charlie saves a stray dog (Scraps) from other dogs. Charlie and Scraps become fast friends and partners in purloining food. Charlie enters a cabaret where dogs are not allowed. Charlie hides Scraps in his baggy trousers, but Scraps' tail emerges from the back end. Charlie meets a girl who works in the cabaret. She is disillusioned with life, so Charlie attempts to cheer her up. Charlie is ejected from the cabaret for having no money and returns to his normal outdoor sleeping spot.
By chance, thieves have buried a stolen wallet nearby that is laden with a small fortune. Scraps digs up the wallet. Charlie returns to the cafe and shows the girl he has enough money for them to be married. The crooks discover that Charlie has the wallet and violently take it back from him. Charlie fights furiously to reclaim it. This leads to a frantic chase which culminates in the thieves' arrest. Charlie uses the money to buy a farm for himself and his bride. The movie ends with the newlyweds peering fondly into a cradle. It contains Scraps and her puppies".
Our pictures today come from the Shop Tour at the Strasburg Railway.