It's a challenge to fully describe the myth of Demeter and Persephone as teaching a moral lesson.  The myth's primary level of explanation or lesson might be more natural than all else in terms of explanation of the season change.  On a more symbolic level, one can use the myth to teach the nature of creation and destruction being one in the same.  The same regenerative forces that end up replenishing the earth and driving its bounty are also the same that bring coldness and a sense of desolation to the Earth.  Demeter must live with the joy of being with her daughter, but is also bound to live with her absence.  She is incapable of living a life that is totally present with happiness and elation.  By the same token, she is relieved of living a life of total despair and misery.  This might be the symbolic lesson to all readers, suggesting that consciousness is complex enough to bridge both the experiences that bring us happiness and the ones that cause us to hurt.  Perhaps, there is a moral lesson here, after all.