Most humour occurs when (two) un-expected contexts are brought together. A context or story is told in the joke and a second context is introduced in the punch-line.
Then we laugh.

So what happens in our brains when we laugh? Perhaps the synapses firing in different areas of the brain are cross-linking in regions of the brain that are not normally active. Or stated differently, synapses that are seldom active are used.
Then we laugh.

We also laugh when we are tickled. Could tickling involve the stimulation of nerve-ends that seldom get sensory input?