Why a good story is (mostly) a good laugh

I'm reading The Light Fantasic, a Discworld novel by Terry Pratchett. It's his second book, written in 1986, when he must have been about 35 years old. I've got to the point (fairly early on) where I don't care what he does with the story, the plot can go nowhere and I'll still be contented, because he has made me laugh. He's demonstrated very witty word play, and some images that are just so damn funny. He's poked a finger at things everyone has been foolish about (like the tooth fairy) and ...