Humor is serious business. Sure, there's simple comedy like a pie in the face or an Adam Sandler movie, but a lot of jokes display real intellect, and despite much reflection and experimentation—both in labs and on stages—no one has yet discovered a unified theory of hilarity.
Stress generally refers to two things: the psychological perception of pressure, on the one hand, and the body's response to it, on the other, which involves multiple systems, from metabolism to muscles to memory.
Anxiety is a normal reaction to stressful situations. But in some cases, it becomes excessive and can cause sufferers to dread everyday situations.
This type of steady, all-over anxiety is called Generalized Anxiety Disorder.