3 ideas
A fox who fails to reach a bunch of grapes walks away declaring they were probably sour anyway. The mechanism is self-protective revaluation: when a goal proves unattainable, the mind lowers its estimate of the goal's worth rather than admit its own failure. The disparagement tells you about the speaker's inability, not about the object.
A hare far faster than a tortoise loses their race because, sure of winning, he stops to nap while the tortoise never stops moving. Superior capacity is worthless if overconfidence lets it sit idle, and a steady rate kept up over the whole course beats bursts of speed with gaps between them. The outcome depends on sustained effort, not on peak ability.
A shepherd boy raises false alarms about a wolf for amusement, and when a real wolf comes, the villagers ignore his cries. Every false signal lowers how much others trust the signaler, and that trust does not recover on the one occasion the signal is true. Credibility is a stock drawn down by each lie, so a liar is not believed even when he tells the truth.