Why do we laugh? "We laugh because it hurts."

I was reading Stranger in a Strange Land by Robert A.Heinlein (if you have no read this book i strongly recommend it, it's an amazing book!).
The character in this book claimed he understood why humans laugh.

"I've found out why people laugh. They laugh because it hurts... because
it's the only thing that'll make it stop hurting...I had thought a funny thing is a thing of goodness. It isn't. The goodness is in the laughing. I grok it is a bravery...and a sharing...against pain and
sorrow and defeat." p289-290

I believe this holds truth. Think about all the jokes you can remember. The 'punchline' or the height of the joke is the when the cruelty, the sadness, or just plain bad is revealed to the listener (or reader).
The same goes for when see someone do something stupid, like walk into a lamp post, or fall over. We laugh.

Do you agree? If not then why? Why do we laugh? What do we find funny?

Laughing is an involuntary reaction, possibly related to survival instinct. It could be for any number of reasons. My personal favourite is as a warning for people nearby. It's changed a bit since then obviously.

It never claimed that it wasn't an involuntary reaction. But think about it, there must be some connection between humor and pain/cruelty?

That's what I was saying. Laughter is believed to be linked to survival instincts. At some point in our evolutionary history, it probably served as a warning.

we laugh when something is unexpected and catches by surprise.