Frequent laughter is a sign of good mental health. Laughing helps a person cope with adversity, and it helps a person enjoy life.
What makes us laugh? A few thousand years ago, Aristotle pointed to surprise. Many jokes have some surprise in them. For example, here is Stephen Wright: "My main goal is to live forever. So far, so good."
It helps us laugh if a joke elevates us relative to others. Political humour plays that role, lowering leaders in our eyes. Some people laugh when they see a photo of Donald Trump with a witless expression.
Schadenfreude also falls in this category. When we see a guy walk out into the ocean on rocks just above the water, and he gets soaked by a wave, we feel smarter or more fortunate than him.
Cleverness helps. How else can we explain puns like these? "A thief attempted to steal paintings from the Louvre in Paris, but was caught two blocks away when he ran out of gas. All the thief could say for himself was: "I had no MONET to buy DEGAS to make the VAN GOGH. But I tried for it anyway because I had nothing TOULOUSE!"
A just result figures into humour. When I was a university student in Colorado, I laughed when I saw a bus unable to get going because a campus road was so icy. As I walked by the bus laughing, my feet slipped out from under me and I fell on my rear. I laughed harder then because of the justice of my fall.
Shared knowledge usually is important to a person making another laugh. When a student told me that I had the personality of Ned Flanders, I had to be aware of that character on The Simpsons to see the humour (and truth) of the comment.
When others find something funny, we tend to copy their response. Thus, comedy TV shows often have laugh tracks to help us judge what is funny. How do we explain tickling-induced laughter? Our lack of control is important, along with having a close relationship with the tickler.
What of nervous laughter? It can result from sudden alarm or confusion. Some individuals laugh when they receive a shock and do not know how to respond. Humanity may end with neither a bang nor a whimper. It may end with a titter.
I may have taken on a thankless task in trying to explain laughter. To borrow words from E. B. White, the author of Charlotte's Web: "Analysing humour is like dissecting a frog. Few people are interested and the frog dies of it."