Others have pointed out the phenomenon, but I like this concrete illustration of it in action.
Slashdot once interviewed The Filthy Critic, and he talked about a friend who had spent time in the writing rooms of a few sitcoms. The writers working on shows that were really pretty poor thought their work was hilarious. They couldn't get enough of their own jokes. He contrasted it with The Simpsons's writing room: full of frustrated, self-loathing writers who didn't think what they were doing was any good at all.
Slashdot once interviewed The Filthy Critic, and he talked about a friend who had spent time in the writing rooms of a few sitcoms. The writers working on shows that were really pretty poor thought their work was hilarious. They couldn't get enough of their own jokes. He contrasted it with The Simpsons's writing room: full of frustrated, self-loathing writers who didn't think what they were doing was any good at all.