The tempting clusters were too high to gain;
Grieved in his heart he forced a careless smile,
And cried ,‘They’re sharp and hardly worth my while.'
The fable follows this pattern: something is desired, it is determined that the thing desired is unattainable, the desired thing is mocked/critisized.
Cognitive dissonance is the uncomfortable feeling caused by simultaneous conflicting ideas. "I want that, but something's keeping me from getting it."
The connection between the spiritual life and the concept this fable illustrates is undeniable. Everyone has a desire for God, holiness, and happiness. But living it out can seem like a goal to high to reach. So to soothe our cognitive dissonance, we mock the thing we desire most but are too weak to have.