Some jokes crack us up. Some fall flat. Why is that?

Why some jokes fall flat -

* One doesn't relate to the subject material.  One is more likely to get a laugh out of a joke about having children from an audience who have/had children than those who haven't.
*  The subject matter goes against one's core beliefs.  Telling an irreverent joke told to a devoutly religious person probably won't invoke much laughter.
* The audience is the wrong age.  Bathroom humor is more funny to juveniles than to most adults.
* The subject matter isn't funny.  If one is telling jokes about ice cubes, they're probably going to get a cold response.