In the fine arts a grin was only characteristic of peasants, drunkards, children, and halfwits, suggesting low class or some other deficiency
Kodak’s extensive marketing campaigns emphasized the pleasure of the photograph, instructing professional photographers to seek out families during holidays to strengthen the connection between celebrations and photography.
as consumers started to buy cameras for their own use, slogans like “Save your happy moments with a Kodak” taught them that the camera was a tool to record joy.
Source: Why people used to look so serious in photos but now have big smiles – The Washington Post