Shakespeare’s speaker feels himself composed of four elements, and here the two lightest ones — air and fire, which he equates with thought and desire — keep darting away to the person he loves. No matter where he is physically, that invisible part of him is always with them. The problem is that when these quick, spirited parts depart, he’s left with only the heavy remnants of himself, sinking into a death-like sadness. Relief comes only when his thoughts and desires return carrying good news of the beloved’s health, and for a moment he feels joy. Yet the comfort never lasts. Almost immediately, he sends those inner messengers back again, and the loss returns, pulling him straight back into gloom. The poem captures a restless, self-perpetuating ache: a loneliness that briefly lifts, only to be chosen again, over and over.
Sonnet 45: The other two, slight air, and purging fire — Summary
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 recur'd By those swift messengers return'd from thee, Who even but now come back again, assur'd, 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.