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.
The opening  quatrain  begins the dual themes of food and wealth as  metaphors  for love and spirituality. These will be followed through the sonnet, to be resolved in the final  couplet . 
 The speaker explains to the Fair Youth that, as he needs food regularly to sustain his body, so the boy is needed to sustain his soul and mind.
Sonnet 75