So are you to my thoughts as food to life.
Or as sweet-season'd showers are to the ground;
And for the peace of you I hold such strife
As 'twixt a miser and his wealth is found.
Now proud as an enjoyer, and anon
The metaphor continues by referring to the means by which food is produced. Just as rain is essential to the growth of crops, so the boy is essential for the Bard to live. 
 This line is an  intertextual reference  that echoes Chaucer’s  “General Prologue” to  The Canterbury Tales : 
 Whan that Aprille with his shoures soote, The droghte of March hath perced to the roote, And bathed every veyne in swich licóur Of which vertú engendred is the flour;
Sonnet 75