The facial feedback theory is a psychological theory associated with the emotional consequences due to physiological actions.
The provided case study a number of smiling faces which made the person saying it smile thereby working there muscles that related with smile this makes a psychological consequence and people find things much more funnier than they usually are.
If as a viewer again this case study is provided and the viewers is allowed to mimic in mirror he or she will again make much funnier faces due to the psychological consequence.