What causes laughter? I was looking at this video from VSauce: "Why did the chicken cross the road?", where several facts about this old joke are exposed and explained.

At some point, (6:59) Michael explains that there is a theory, based on the paper Computer Model of a "Sense of Humour" by I. M. Suslov, that says that laughter is the result of the released energy on the pre-judgement one makes about the result of the joke.

Still, this theory does not account for factors like being under the influence of drugs (on which it is generally easier to laugh at things), situations like tickling or other forced laughters like laughter gas itself.

So, which is the currently, widely accepted theory that explains what causes laughter?
The majority of researchers believe it has to do with relief to the flight or fight system and related to human bonding. 

For example, we hear the phrase "laughter is contagious". That's because we want to let others know we have good feelings towards them and would like to connect at some level but in a quick and non-direct way.

However, for situations where the joke may be "dangerous", Suslov's theory is better understood. I have also personally witnessed it many times. This kind of laughter follows the familiar model: "Oh no, I hope he doesn't say something that pokes fun or embarrasses me in this social setting". Next, you hear the joke and realizing it doesn't embarrass you (too much) the relief and release of energy transmutes into a laugh.

Also see: http://en.wikipedia.org/wiki/Laughter#Causes

Reference: 
How Laughter Works:
http://science.howstuffworks.com/environmental/life/inside-the-mind/emotions/laughter.htm