concerned primarily with a romantic theme. The sonnet contains fourteen lines with three quatrains and acouplet at the end. Each of the fourteen lines of an English sonnet is composed in ten syllables of iambic pentameter which means that each line has a pair of syllables, an unstressed followed by a stressed syllable. The traditional rhyme scheme is ABABCDCDEFEFGG. The ninth line is called a turn because the subject shifts slightly.

The tone of the poem is apologetic and reassuring. Thematically, the sonnet emphasizes the affection that the poet holds for the young man. With his absence the fair lord may have felt that the poet’s love had waned. 

The narrator denies that he has any insincerity in his affection for his lover. Three times the poet declares that no matter where he may travel — both physically and in his thoughts — he will always return to the youth, for the young man is his alter ego

The sonnet seems to spring from an accusation of unfaithfulness. An accusation which the poet denies. Lovingly, he explains that he can never separate himself from this young lord.

To leave for nothing all thy sum of good:

For nothing this wide universe I call,

Save though, my rose; in it thou art my all...

Admittedly, the poet has sensual feelings; however, how could his young man believe that he would act on those lusts if it would separate the two of them. The universe is nothing unless the poet has the fair lord there with him.

Shakespeare used the metaphor of the flame to represent the warmth of his love for his lover. Another comparison points up the lover’s breast as the home where he keeps his soul and love. Alliteration was used to add to the rhythm and emphasize certain points:

Never believe, though in my nature reign'd

All frailities that besiege all kind of blood...

As in all of his sonnets dedicated to the fair lord, Shakespeare makes clear his love and admiration for him. 

The speaker of this sonnet is lovesick and in the poem reflects on how this love has affected him. In the first line he uses a simile ("My love is as a fever") to compare the love to an illness, and the next part of the sentence ("longing still / For that which longer nurseth the disease") also suggests that the love is like a compulsion. The more he longs for, or desires, the object of his love, the worse the "disease" becomes.

The next line ("Feeding on that which doth preserve the ill") suggests that the love is like a parasite. It feeds on and exacerbates his illness without giving him any succor in return. Or, alternatively, his love is like an insatiable glutton. It has an appetite that can't be satisfied, and the more it feeds, the hungrier and more gluttonous it becomes.

In lines five to seven, the speaker says that his love has made him irrational. His reason, personified as "the physician to (his) love," tells him to stop this self-destructive love, but is ignored. Consequently his reason has left him, although apparently not entirely, because he still has reason enough to realize that "Desire is death." In other words, he realizes that his desire for the object of his love is killing him.

In the final six lines of the poem, the speaker declares that his lovesickness has become so severe as to be "past cure." He also states that his "reason is past care." The implication here is that the rational part of him is fed up with being ignored and is no longer willing to advise against the love only to be continually ignored.

The speaker also, in these closing lines, compares himself to a madman. Reason has left him and, left without reason to temper it, his love has made him "frantic-mad." In the final two lines, which form a rhyming couplet, the speaker offers a definitive explanation as to why his love has caused him such anguish. He directly addresses the object of his love and says:

For I have sworn thee fair and thought thee bright,
Who art as black as hell, as dark as night.

The juxtaposition here, between "fair" and "bright" on the one hand and "black," "dark," and "night" on the other indicates that the speaker can't help being in love with someone undeserving of that love. It is thus his inability to reconcile his love with his reason, as alluded to previously in the poem, which is at the root of his anguish. He loves someone undeserving of his love but still can't help but love them. He is like a drug addict who knows that the drug is harmful but can't help but continue to take it.

The narrator is speaking of the intense desire when love turns into infatuation.  It is a dangerous time, when a person can turn against his or her self. 

Desire is death, which physic did except.

This is partly what is meant by “Desire is death,” because the speaker is acknowledging the damage of being so far gone in lust.  He describes himself as “past cure” and “frantic mad.”  These are strong words.  He is becoming mad from the tension of love and his inability to control himself.

Past cure I am, now reason is past care,

The references to “physic” and the medical metaphor demonstrate how love can become like a sickness when it is an infatuation.  There is no way to get out.  This is partly the reason for truth “vainly express’d,” because even acknowledging the infatuation does not rescue one from it.

This sonnet explores the darker side of love, when a person gets in over his or her head.  Passion and infatuation can make a person feel out of sync with his or her self, and not in control of his or her faculties.  It is scary to lose control, but that is what we do when we fall in love.

In the beginning of “Sonnet 147,” the speaker compares love to a disease.

My love is as a fever longing still,

Although this does not sound like a very nice comparison, the metaphor is apt in that we lose control of ourselves when we are in love, just as we do when we are sick with fever.  Just as a fever makes us delirious and confused, love does the same.

For that which longer nurseth the disease;
Feeding on that which doth preserve the ill,

The longer you have a fever, the sicker you get.  The longer you are in love, the more in love you get.  In either case, it is difficult to get out from under once you are under.

The uncertain sickly appetite to please.

Love and infatuation are self-propagating, just like a fever.  If there is no intervention, a fever feeds on itself.  Likewise, in love if there is no intervention the love-sickness gets stronger and stronger.

My reason, the physician to my love,
Angry that his prescriptions are not kept,
Hath left me, and I desperate now approve

When we are in love, we do not often think with our heads. In fact, sometimes we cannot believe what we are doing.  We know better, but we are infatuated.  We cannot help ourselves.  We make choices out of love, and not out of logic.