Humor is a very subjective thing. What makes one thing funny and another not funny is based largely on the person, I think.

Take "poor taste" jokes. For example, how do you fit 44 Jews into a VW Bug? Two in the front, two in the back and 40 in the ashtray. I happen to think that such a joke is in poor taste and hence it's not really that funny to me. By contrast, I know some who would laugh uproariously at it. The joke is the same; what changed is the person's perspective. Not everyone appreciates the same type of humor. My former youth minister would crack "corny" jokes all the time and I was the only one in the youth group who would laugh at them; everyone else would just groan and roll their eyes. He and I were alone in our like for those types of jokes. But why? I don't know.

I used to have a half-baked theory that humor comes from that which you enjoy, arriving strangely.

Not surprise. Incongruously would be a better choice of words. Think of it this way-- As we grow and learn, our brains build up a mental model of how the world around us works. We make millions of interconnections that allow us to efficiently exist in the world without having to analyze every new situation from base principles. Our minds become elaborate machines constructed out of interlocking ideas and knowledge, with every input having a roughly defined path through the machine.

Incongruity results when an idea manages to take a nonstandard path through the machine. Laughter is your brain being tickled by the idea forcing its way along the wrong path. But the idea and the path have to add up to something you like for there to be laughter. Otherwise the result can be confused indifference, or the opposite of laughter-- shock, horror, disgust.

Consider physical violence. Most of us have a healthy appreciation for violence. Young and/or stupid people can be made to laugh by something as simple as unexpectedly punching someone in the face. But more developed minds have already accepted and internalized random pointless violence as just one of those things that happens, so they require more elaborate, less obvious setup. A pie in the face. One of Wile E. Coyote's doomed devices. Dr. Strangelove. Etc. Babies laugh at peek-a-boo because they haven't learned object permanence yet. Adults laugh at peek-a-boo because what the hell dude why are you doing that? I like jokes that require you to think a bit because I like thinking. Etc.

Yeah, that was an impressive description. Now explain running gags.

Or, to keep with your previous explanation, the non standard element about a running gag isn't the gag, but the different situations in which it unexpectedly appears.
It's coming from a new direction every time and may gain new meanings from these (taking a new path through the machine). Usually a running gag isn't a full joke anyway, but a catchphrase or some other short utterance that's fit to take on different meanings in different circumstances.

EDIT: The pleasant part would be group confirmation, as running gags are only funny with people who know that gag.

Now what about the necessary existing knowledge that we spoke about before?
I'm not sure if it applies to all humour, but generally there's got to be a known element and an unknown/unexpected element. This could be seen as a premise for an idea to take a nonstandard (wrong) path through the machine: If the idea is completely new it cannot deviate from an existing path but will take a new path all along, which somehow isn't quite that funny.

Isaac Asimov wrote a short story about humour, where he theorised that it was really some kind of bizarre experiment that aliens were performing on the human race to test them.  The story is called "Jokester" and was published in "Earth is Room Enough".

Some of what Samuel Langhorne Clemens (Mark Twain) wrote / said on humor:

"Everything human is pathetic. The secret source of Humor itself is not joy but sorrow. There is no humor in heaven."

"Laughter without a tinge of philosophy is but a sneeze of humor. Genuine humor is replete with wisdom."

"Humor is the good natured side of a truth."

"...humor cannot do credit to itself without a good background of gravity & of earnestness. Humor unsupported rather hurts its author in the estimation of the reader."

"What is it that strikes a spark of humor from a man? It is the effort to throw off, to fight back the burden of grief that is laid on each one of us. In youth we don't feel it, but as we grow to manhood we find the burden on our shoulders. Humor? It is nature's effort to harmonize conditions. The further the pendulum swings out over woe the further it is bound to swing back over mirth."

"American humor is different entirely to French, German, Scotch, or English humor. And the difference lies in the mode of expression. Though it comes from the English, American humor is distinct. As a rule when an Englishman writes or tells a story, the 'knob' of it, as we would call it, has to be emphasized or italicized, and exclamation points put in. Now, an American story-teller does not do that. He is apparently unconscious of the effect of the joke."


Then there is:

“The secret to humor is surprise.” - Aristotle

"The wit makes fun of other persons; the satirist makes fun of the world; the humorist makes fun of himself." - James Thurber

"Comedy is nothing more than tragedy deferred." -  Pico Iyer

“Humor is reason gone mad.” - Groucho Marx


And finally,

"Defining and analyzing humor is a pastime of humorless people." - Robert Benchley

There's one aspect of humour that we haven't really talked about yet. And that's how different types of humour can reveal or express a certain mindset. The mindset behind a minority joke is quite obvious but there's a more general description. The thesis is that there are basically two types of humour:

1. Conservative Humour
This type implicitly defends accepted notions, values and rules by showing how an unfit person or idea comically fails at them, often by exaggerating traits of that person or idea. This will help strengthen cohesiveness of a group by defining the group negatively (saying what's not accepted in the group).

2. Radical Humour
Conversely this type of humour attacks accepted opinion, values and rules by introducing an unusual viewpoint and reinterpreting them from this point. Example: "If the poor weren't so ugly the problem of poverty would soon be solved." (Wilde)

I assume there could be just as many categorisations like these as there are general perspectives on society but I found these to be especially interesting and helpful to quickly determine for myself what it is I like or don't like about a joke.

As you may have noticed the humour theory presented by ZylonBane only describes Radical Humourdoesn't describe Conservative Humour, which is fine with me because Radical is my preferred type. But it's interesting to note anyway.

gotta keep the mind occupied, or else it will rot away. topics like this one are perfect for that.

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