Zach Weiner in The Weinerworks:
Warning: Evo psych just-so story to follow. Think of it as a parable, not as a theory. It’s just here to contextualize the idea that follows.
Suppose there’s a monkey. Suppose also that the monkey has evolved to have an inbuilt proto-toolmaking behavior.
For this specific example, let’s say he’s learned to snap a twig off a tree and stick it in an anthill. When he pulls it out, it’s covered with tasty protein-rich ants.
This monkey is unlike you and I in that he takes no pleasure in finding the right stick. He knows the stick must have certain qualities – long, thin, not too brittle. However, he does not experience any pleasure until he actually eats the tasty ants.
Suppose this monkey represents a species. This species does well because it has this one trick for getting protein out of the ground in abundance at a low cost.
Now, suppose one day a monkey is born who has a quirk. Instead of taking pleasure in the ant part, he takes pleasure in the stick selection part too. That is, when he finds an appropriate stick, his brain rewards him with premature pleasure. So, whereas his brethren experience pleasure only upon eating the ants, this one monkey gets pleasure from selecting an appropriate branch.
More here.