Why do people smile in photos?

Snsns
I don't get why almost every photo I see of someone they are smiling... What about a camera lense is it that makes them laugh or feel happy?
When I'm taking a picture I never feel like smiling... I feel blank so I never smile in photos... how comes every one else is so happy when taking pictures? I don't get it... What is it about taking a picture that fills them with such joy?
Why do people smile in photos?
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