Laughter is one of the most inexplicable facets of human behaviour. Many theories have been put forward to explain it but none, as far as I can see, have reduced the nature of it to a simple explanation.
The first question to ask is why does it exist at all? What possible purpose does it serve?
Clearly, for animals with complex social structures it is necessary to have a mechanism with which to identify favourable members of a group. Other primate species exhibit such behaviour, such as Baboons who groom the fur of others in order to curry favour with them and win acceptance.
What would human society be like if we didn't laugh? It is hard to imagine; however, one could probably surmise that it would be more difficult to identify like-minded people with whom one could feel relaxed and safe, and therefore people would feel less comfortable within groups. When attempting to be funny one is essentially inviting others to participate in a shared experience that boosts the standing of the person making the joke, and it breaks down barriers of mistrust and uncertainty between all those laughing along with them.
Given that socialising can be difficult and fraught it is as though a compensatory pleasure evolved to give happiness to people when they have successfully bonded with and been accepted by a group. Without laughter there would have to be something else to serve that purpose.
What makes something funny and why do we laugh at those things?
I was prompted to think about this subject after I identified a pattern among the jokes of a friend of mine who was particularly good at making people laugh. My conclusion is that something is funny when it is simultaneously logical and illogical.
In addition, though, is the crucially important delivery of the joke. In my MM piece "Orchestration and Aesthetics" I point out that the way a joke is told is more important than the joke itself (much as how in music the orchestration of a tune is more important than the tune itself). Some people have a naturally humourous disposition and story-telling ability which heavily influences how funny something is. Humour only seems to work when in the context of a moment because it exists to bring unity and calm to a tense social situation. A joke out of the context of a given moment isn't particularly funny. People with comedic quality seem to be able to tell jokes with a rhythm and vocal inflection in a way that captures the essence of a conversation and continues it.
But why did we evolve to laugh at things that are simultaneously logical and illogical? I once saw something on the TV where a reporter was stood with his back to a Chimpanzee enclosure. A Chimpanzee came over and poked the reporter, who jumped in surprise, and the Chimp put on a facial expression reminiscent of a huge smile. It seems, therefore, that humour in a primitive form exists among Chimpanzees who do not possess language. The Chimpanzee was mocking that person without being overtly harsh towards them. When carried into language the use of conceptual incongruities form shared positive understanding and enable group bonding without too much aggressiveness. It would seem that humour evolved from Apes into Humans out of the necessity of group bonding whilst avoiding too much conflict.
It has often been said that humour involves ridicule. If the only way to bond a group was by being nasty to other groups then humanity might not have survived. I believe humour serves to bond a group in a way that avoids overt nastiness. If someone is being annoying within a group then making a joke about them is a way of changing their behaviour without being aggressive. Humour elevates the social standing of a person by pointing out a clever incongruity relevant to the situation at hand. People laugh at it and this serves to bind the group, often involving put-down, but in a non-aggressive manner.
Logical incongruities are a natural part of any language-driven understanding of the world and evolution found a niche with this for human social interaction to help us live in groups without our species tearing itself apart.