The governing metaphor of this sonnet is the way that, in the first line of the poem, the love that the speaker is suffering from is compared to a fever, that the speaker deliberately tries to prolong and keep even though it is making him sick:
My love is as a fever, longing still
For that which longer nurseth the disease,
Feeding on that which doth preserve the ill,
The uncertain sickly appetite to please.
Clearly, the relationship that the speaker has with his beloved is not something that is healthy and productive, and he feels that it has reached the point that he is so in love with his beloved that it has turned into an unhealthy kind of love that is now wounding him instead of sustaining him.
Reason, who is described as the speaker's doctor, is battling in vain to try and counsel the speaker to stop this relationship. Having failed in this job, he has abandoned the speaker to his fate and leaves him in a terrible state, as he is in a position of near-lunacy. The final rhyming couplet of the sonnet states the reality of the situation. Blinded by love, the speaker thinks that the object of his affection is lovely, even though he or she is "as black as hell, as dark as night."