"To tease something desirable, but keep it constantly out of reach."
A man is riding a pig by dangling a carrot on a stick in front of the pig's nose. The pig chases the carrot, never coming close to eating it. The man riding the pig has a destination to ride to, so to keep the pig from losing motivation, he never gives the pig the carrot.
The pig doesn't realize that it is a slave to its rider and that its reward is always out of reach. It can't free itself from its torment because it is insensitive to it.
This metaphor seems to be prevalent for humans: the American Dream, losing weight, getting drafted into a war1, paying off a mortgage/credit card interest/student loan, getting promoted at work, living up to older siblings, etc. Some of these campaigns2 are unattainable and serve only to tantalize us with a reward that may/will never come.
We're not aware of the agony of chasing these campaigns that motivate us to wake up in the morning. These are not goals, hopes, or dreams. They're carrots on a stick, and we're the pigs.
Who's the rider?