One theory to explain humor is that it is the mental state of "flipping" between two or more conceptualizations of the same thing. You find input humorous when it is unexpected and different from your current model of reality, and you are forced to add that new input to your list of expectations. The effect relies on your imagination in knowing what will probably happen, and your confusion when it doesn't happen. Whether or not something is funny depends not only on how unexpected that input is, but on your internal state and thoughts about the input.

Thus when a school bus randomly explodes on the Simpsons, it's funny, whereas when a firework does the same thing in Real Life, it is not. This also explains why a stimulus that is humorous the first time is less so, or not at all, the second. This is most noticeable in very predictable humor like the Three Stooges or TV sitcoms. If a humorous input is sufficiently complex or draws upon concepts that aren't often referenced in daily life, it can be funny more often, as is the case with Monty Python and SOY!. There is a point of diminishing returns with regard to complexity and obscurity, however, as most people just don't find jokes told in Calculus notation or about Millard Fillmore very funny. Dependence upon forethought and imagination also explains why dull people often lack a sense of humor.

Another facet of this theory is that your physiological, emotional, and cognitive states can "block" you from making the associations and preconceptions necessary to find humor in a situation. When you walk into a pole you don't see, you don't laugh because you are busy being in pain, but when you look back on the incident in a week, it's hilarious and you can laugh along with your friends at the memory. Likewise, when you're crying about something, you are probably using the whole of your concentration and emotional depth on that thing, and cannot or are not willing to spend the thought needed to appreciate a funny stimulus. Humor can also be interfered with by a deeply held belief or opinion, which is why the strongly Christian find no humor in jokes about Jesus. Similarly, it's hard to laugh at yourself, because deep down we don't consider ourselves buffoons, and are not willing to examine the possibility that we might be.

It should be noted that under this theory, when we find something funny, we incorporate it to a small degree into our entire thought process. In essence, we learn it. That's why upon reading Hitchhikers Guide the second time, we take almost as much pleasure in anticipating the humorous parts as we do in reading them. That's also why racist/sexist humor is so insidious, because in laughing at it, we incorporate it into our psyche as a plausible (although not necessarily true) fact.

For more on this theory, see:
Belief and the Basis of Humor, Hugh LaFollette and Niall Shanks, American Philosophical Quarterly, 1993, 329-39
Laughter, Robert R. Provine, American Scientist, 1996 Jan-Feb

There's some pretty good evidence that we consider situations funny when we confuse frames of reference; that is, when we expect a certain context for a remark or phrase but get a different one. Consider:

"My best bloodhound's got no nose."

"Really? How does he smell, then?"

"Terrible."

The expected context is fairly obvious for that one; it's a bit more subtle for most jokes, but it's still usually there. That explains why a joke isn't funny the second time you hear it, too; you know what frame of reference to expect, so your brain's context handlers don't get confused.

(Enth's node, above mine, has more on this)

Now, obviously some way to distinguish between confused frames of reference is necessary; we aren't perfect, and what with the way our system of perception works, confused frames of reference are certain to be a fairly common occurrence. Give a good joke to a robot, and it might stop working, muttering something along the lines of "syntax error" (for a fictional example, read Wizard and Glass, the fourth book in Stephen King's Dark Tower series). Give one to a human, and s/he starts laughing. Pretty decent solution, if you ask me.

A more interesting question is why we consider funny situations pleasant. By definition, a funny situation is one that we don't fully understand; why, then, do we not only not mind one when it happens to us, but actively seek them out and distribute them to others?

This is just a theory, but I think it has to do with the nature of our (continuously developing) intelligence. A funny situation isn't just incomprehensible; it's comprehensible enough that it can nearly be understood, but strange enough that our internal parsers don't handle it perfectly. For a system that wants to learn new things, a situation like this would be a godsend. In the real world, understanding of something funny would lead to greater understanding of the world at large, essential to a species (us) whose evolutionary ticket to success is high intelligence. That's probably why some Zen koans are funny; they're designed to force a new way of looking at the world, and they do so by confusing frames of reference. There's a saying along the lines of "If it makes you laugh, it might be true". Perhaps this is the truth behind that saying.

Since our species depends so much on becoming enlightened (or at least elucidated), it makes sense that we would appreciate situations that help us to that goal.

Hu"mor (?), n. [OE. humour, OF. humor, umor, F. humeur, L. humor, umor, moisture, fluid, fr. humere, umere, to be moist. See Humid.] [Written also humour.]

1.

Moisture, especially, the moisture or fluid of animal bodies, as the chyle, lymph, etc.; as, the humors of the eye, etc.

⇒ The ancient physicians believed that there were four humors (the blood, phlegm, yellow bile or choler, and black bile or melancholy), on the relative proportion of which the temperament and health depended.

2. Med.

A vitiated or morbid animal fluid, such as often causes an eruption on the skin.

"A body full of humors."

Sir W. Temple.

3.

State of mind, whether habitual or temporary (as formerly supposed to depend on the character or combination of the fluids of the body); disposition; temper; mood; as, good humor; ill humor.

Examine how your humor is inclined, And which the ruling passion of your mind. Roscommon.

A prince of a pleasant humor. Bacon.

I like not the humor of lying. Shak.

4. pl.

Changing and uncertain states of mind; caprices; freaks; vagaries; whims.

Is my friend all perfection, all virtue and discretion? Has he not humors to be endured? South.

5.

That quality of the imagination which gives to ideas an incongruous or fantastic turn, and tends to excite laughter or mirth by ludicrous images or representations; a playful fancy; facetiousness.

For thy sake I admit That a Scot may have humor, I'd almost said wit. Goldsmith.

A great deal of excellent humor was expended on the perplexities of mine host. W. Irving.

Aqueous humor, Crystalline humor ∨ lens, Vitreous humor. Anat. See Eye. -- Out of humor, dissatisfied; displeased; in an unpleasant frame of mind.

Syn. -- Wit; satire; pleasantry; temper; disposition; mood; frame; whim; fancy; caprice. See Wit.

 

© Webster 1913.

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Hu"mor (?), v. t. [imp. & p. p. Humored (?); p. pr. & vb. n. Humoring.]

1.

To comply with the humor of; to adjust matters so as suit the peculiarities, caprices, or exigencies of; to adapt one's self to; to indulge by skillful adaptation; as, to humor the mind.

It is my part to invent, and the musician's to humor that invention. Dryden.

2.

To help on by indulgence or compliant treatment; to soothe; to gratify; to please.

You humor me when I am sick. Pope.

Syn. -- To gratify; to indulge. See Gratify.

 

© Webster 1913.