It really depends on the person being photographed. As others have said, maybe they don’t want to be photographed, or perhaps they’re trying to be silly. Maybe they don’t know how to pose for a photo and their default is being silly and, well, :P
All I know is I prefer it to other faces. My younger cousin (7 y/o) in particular annoys me with the strange faces and poses he makes in an attempt to be cool. His favorite is to look angry/tough with bulgy eyes while posing in strange positions and giving hand signs. I yell at him, “give a good smile and look ADORABLE!”. But, I digress…