A lot of jokes are funny because they are the opposite of something we might expect. So where in the brain are we creating these fanciful links ?
That's theoretically correct, we experience laughing moments of any kind it's because of relief from opposite expectations. Google defines that there is a general theory that explains laughter is called the
relief theory. I didn't really look much into this theory, but it explains very well of the causation.
"Humor may thus for example serve to facilitate relief of the tension caused by one's fears. Laughter and mirth, according to relief theory, result from this release of nervous energy."While the mechanism of laughter within humans have been long debated with multiple theories, however.
https://en.wikipedia.org/wiki/Theories_of_humorAnd for a punch line, why are jokes funny and why to some people not.
That I believe it has to do with our experience within our life. All of us have different viewpoints, and range of knowledge, limited or different taste in sense of humor, and such. Some won't understand, some do. It entirely depends on the mentality of oneself.
And then I wondered where the joke came from. I don't remember making up jokes when I was little. It must have been something I learnt during some period of my life. So it's probably a learned thing or is there a part of the brain that deals with the often absurd. Hmm.
I always fails to understand jokes because I am not sociable, I have hardly understood the experience of moments with anyone properly.
Therefore, even if one have seen and says;
"A big skinny bald guy fell off stairs and rolled like a tumbleweed." -- Since I've never seen how it is like, I can't really fully grasp what is there to be relieved. While I could only imagine how sorry for this skinny guy falling off stairs like that, he could have broken his bones.
Oh, but that case is different for fatties! Oh boy hoo boy
*Frantic laughter* 
Just kidding, I'm sorry.
And discuss.
I discovered that figuring out what is there to laugh about relies on understanding on the viewpoint, and the manipulation of our emotion-recalling mechanism based on the subject that one is looking at. Of course if one just says;
"A bald guy fell off the stairs." sounds cruel. But the part that makes it funny is the
"fell and rolled like a tumbleweed", because that is the focal point of hilarity.
In common sense, the memory of
"rolled like a tumbleweed" has no emotion value to anyone. Because it's just a tumbleweed. If one reads from left, to the right, one would attempt to recall from "sadness" into nothing to be worry about.
"He rolled like a tumbleweed" attempts to cause the reader to relate between each objects, this perhaps causes our emotional viewpoint to shift from the person into a tumbleweed instead. That fits the theory of relief. That is, unless one have a different emotional viewpoint for the tumbleweed.
I believe this is the same for the jokes. One asks a question that generates a certain tension, and then a ironic answer is given that blows one's mind off into unexpected relief, and that is what gives us the sense of humor.
For additional example, let's say I told you a sarcasm that can be easily regarded as something terrible, or perhaps one just does not realize that it was a sarcasm, which means it is not real.
To counter that, one can further express that one is actually just joking to relieve the situation, with just a word or two.
The amount of relief and laughter is based on how much tension and how much that tension is then taken away. We sure have the ability to change our emotional viewpoint pretty fast.
