1 Making fun of someone they don't like
People who don't like certain political candidates laugh more at jokes told about them. People who like that candidate might not find the joke that funny and they might even find it offensive.
2 They are unexpected
Jokes with the most unexpected twists are generally funnier than predictable ones. Humans love unexpected things because they get them excited and find predictable things boring.
3 They depend on the person's beliefs
The reason people perceive jokes differently is that each person has his own unique belief system, a big part of a joke's effect depends on its interaction with a person's belief system.
4 They help people satisfy certain needs
Jokes can help people satisfy certain needs that aren't satisfied otherwise. People usually find jokes about things they really wish to do funnier than other jokes.
5 They help people compensate
A man who doesn't think he is that masculine might become very interested in spreading adult jokes. In such a case, those jokes help the man compensate for his perceived weakness.
6 Release psychological tension
Jokes can help people release psychological tension in a safe and fun environment. Jokes can help people cope with big problems. This is why so many people tell jokes about their own problems (See also why we laugh inside elevators).
7 They support people's beliefs
Jokes that support people's beliefs make them laugh more than jokes that don't. People love to be approved and acknowledged and jokes can sometimes be a method to help them achieve their goals.
8 They help people vent
If a person hates something, the traffic jam in his city for example, then a joke about it will help him vent. Jokes can help people vent their suppressed emotions and so make them feel good.