The other two, slight air and purging fire,/Are both with thee, wherever I abide;/The first my thought, the other my desire,/These present-absent with swift motion slide./For when these quicker elements are gone/In tender embassy of love to thee/,My life, being made of four, with two alone/Sinks down to death, oppress'd with melancholy;/Until life's composition be recured/By those swift messengers return'd from thee,/0 Who even but now come back again, assured /Of thy fair health, recounting it to me:/ This told, I joy; but then no longer glad,/ I send them back again and straight grow sad

This is Shakespeare's Sonnet 45.

In this sonnet, he finds himself in continous fluctuation between joy and sorrow. This is as a result of the absence of the one he loved. His thoughts were always moving to and fro between him and his love.

Actually, this sonnet relates to his previous sonnet. He finds that his thoughts and desires are not so much in himself, as with his beloved (hence present-absent.)

Substances were said to be made up of fire, air, earth and water. But when a substance is deprived of two of them, air and fire, (the other two) which correspond to thought and desire, the body responds and sinks into melancholy and decay.