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.