Superiority theories say that humour illustrates the inferiority in some respect of the joke’s butt, provoking laughter as a sort of small triumph in the superior witness. This works well in some cases, but struggles to account for “butt-less” humour such as puns, or the kinder forms of imitation. Release theories have a Freudian pedigree: humour provides a sort of relief from a build-up of nervous tension... Incongruity-resolution theories are more popular: they assert that humorous situations involve the presentation of an incongruity that is subsequently resolved.
I would offer a confidence theory: humor trades on the credibility and sympathy of the joke's creator and audience--their shared understandings, their common norms. This produces some of the gratification described by the superiority theory without requiring that anyone be inferior. Puns, as Lewens notes, are a joke without a butt (except, arguably, the language itself), but they do require mutual literacy between the person telling the joke and the person hearing it, which is nice.
If you consider that all people are in a chronic state of mild nervous tension (modernity and its discontents, the disjunct between the head and the heart, the unsustainable struggle between the will to faith and the will to reason) then this becomes a unified theory in which a joke offers a small splinter of release from the incongruity in which we inevitably live.