Well, it's gross and at the same time funny. If it's accidental - it's funny. Like, oops! If it's like on purpose for fun, than maybe not so much. If it's stinky and accidental - I would not laugh. I would feel embarrassed for them. If it were an all-out explosive bowel invasion - I would feel bad too.
Sometimes, though, guys like to let it out blast french furnace style in your face! Not fun.
Not everybody laughs at that, some find it offensive. It just is funny! Something that is private so when we hear someone we laugh cause better them than us!
Low humor, pratfall, slip, embarrasing, say something inappropriate type of humor. Man's humor mostly. Most men find fats hilarious!
I disagree that over eating mostly causes farts. Everyone -- fat,thin, athlete, couch potato, old, young, black or white, ... farts at one time or another. The funniest ones are the ones that kind of sneak up on you. Yes, boys and men get a bigger kick out of them them than females. <br />
I grew up in a family where my mother never will admit having one and refused to say the word fart. It is uncouth to her and she uses the phrase: "passing gas". We are supposed to just ignore things like that.<br />
Funny movie clip: http://www.youtube.com/watch?v=vkAymkcZO48 from the movie: Love and other Disasters.
It seems that it is moment of vulnerability....in some sense. I try not to blow in public, but if nature dictates, everyone can kiss my smelly ***.
There's a film by Louis Bunuel where dinner guests all sit around the table with their pants down on toilets then, one by one, get up and go to a little room where they eat the meal in private. Most of the film is like this and it's really funny for the same reason. I think it's called "The Discrete Charm of the Bourgeoisie"