From Science Daily:
"The "click" of comprehension triggers a biochemical cascade that rewards the brain with a shot of natural opium-like substances, said Irving Biederman of the University of Southern California. He presents his theory in an invited article in the latest issue of American Scientist.
"While you're trying to understand a difficult theorem, it's not fun," said Biederman, professor of neuroscience in the USC College of Letters, Arts and Sciences
"But once you get it, you just feel fabulous."
The brain's craving for a fix motivates humans to maximize the rate at which they absorb knowledge, he said.