We the People....
They're probably afraid because they're probably afraid of either peeing their pants or blowing milk through their nose.Probably.
Lol. Maybe thats it. Maybe they have a fear of blowing something out of their nose when they laugh. Thats mean. Sorry, I shouldn't make fun of other peoples fears.
That sucks. Those people are missing out on a lot of good comedy movies. haha.
What happens if they see something funny. Do they bite their lip to keep from laughing?