Because they are unexpected
One of the basic elements of humour is something being unexpected. This is why so many jokes have funny twists in the end. Farts are totally unexpected, in most cases, and this is one reason why they can be funny.
Humans use humour to hide embarrasement
Humour is one of the coping mechanisms humans use. People use humour to reduce the intensity of other unwanted emotions such as embarrassment. When a person is embarrassed because he farted he can quickly use humour to feel better.
They break the rules
Many jokes are funny because they help people meet certain psychological needs indirectly. A fart usually breaks the rules and so asserts the desire to rebel especially for kids.
It’s more fun if people can’t escape
A fart is usually funnier if people can’t escape. If for example it happened inside a car or an elevator then the one who did it might find it very funny and because humour is contagious others might respond with laughter.
When Farts aren’t funny
Farts can be considered embarrassing or humiliating when they happen in public places or when the person is surrounded by total strangers. Farts are usually considered funny when they happen among friends. (See Why novel and bizarre things catch your attention?)
Why do boys find them funnier
While both boys and girls can find farts funny still the universal stereotype set for girls, that they should be polite and shy, can prevent girls from laughing at farts as much as guys do.
Farts can sometimes be mysterious because it can be hard to tell who dealt it, especially that sometimes people keep denying that they did them.
There are so many jokes about farts
‘Who smelt it dealt it’ and many other similar jokes have conditioned people to find farts funny. The fact that farts are connected with fun in many occasions make them funnier to people. (See What makes a joke funny?)
Sometimes farts help the person achieve the unconscious desire of punishing some of his friends or taking revenge against them. This usually happens with kids. The fact that the fart helps the kid meet a goal makes it more fun.
They are immature and not serious
Farts can release stress because they can be considered an immature behaviour. When a person does something immature without harming anybody, he reliefs some of the stress of being so serious all the time.
It depends on the social context
The same person can find a fart very funny in one situation but totally inappropriate in another one. The social context and the setting can affect the way people perceive a fart.
Because they come from the bottom
The fact that farts come from the bottom makes them more funny than burping which comes from the mouth.
Why are farts funny?
Why do farts sound funny? Why are farts funny? Why do you laugh when you fart?