What is the main lesson of The Little Prince as allegory?

As an allegory Le Petit Prince by
Antoine Saint-Exupery expresses lessons of friendship and altruism.  Saint-Exupery once
wrote, etre homme, etre responsable [to be man is to be
responsible], and this expression of man's purpose is the philosophy of Saint-Exupery's
The Little Prince. The relationship that the little prince has with
his rose on the planet is pivotal to the novel as the prince learns that it is his
responsibility to the rose, rather than his love for its beauty. In fact it is this
responsiblity that drives him back to the planet and that gives his life meaning.  The
prince also learns that altruistic gestures are more rewarding than selfish
ones.

In Saint-Exupery's allegory there are unnamed
characters who symbolize certain phases of human life.  For instance, the king
represents authority, the businessman respresents greed, and the lamplighter respresents
devotion to duty. The flower is a flirtatious woman, the serpent is death, the fox
represents trickery.  For instance, it is the fox who teaches the prince about the
importance of one's responsibility to the loved one.  He explains to the prince that by
taming him, the prince has invested himself in the fox, thereby making the fox more
special to the prince.  Thus, what one gives to the loved one is more important than
what one receives in return.  Because of this lesson, the prince decides to return to
his planet where his responsibility, the rose, needs
him. 

The links below will connect you to another question
and another site on Saint-Exupery's novel which may be of help to
you.