Smiling in photographs is a relatively recent phenomenon, as you can see from this and many other photographs in the JoCoHistory database. In his 2003 book A Brief History of the Smile, Angus Trimble attributes this to everything from slow shutter speeds to poor dental hygiene to the advent of motion pictures in the earliest decades of photography. Regardless, smiling seems to have come firmly into fashion by the 1930s.
The two gentlemen above are unidentified. The picture comes from an early 20th century scrapbook. Click the photo to view larger image in the photo database.