My love is as a fever, longing stillFor that which longer nurseth the disease, (1-2)
Comparing one's love to an illness has got to be one of the biggest clichés ever. But we don't think the speaker is talking about love here. When he says it's like a "fever," we get the impression that he's talking about the way he's burning up with lust.
The uncertain sickly appetite to please. (4)
We've heard an appetite for food being a metaphor for sexual desire but, this is kind of gross. When the speaker calls his appetite "sickly," he makes it pretty clear that he thinks his lust is unhealthy, to say the least.
Desire is death, […] (8)
Theoretically, sex between a man and a woman is supposed to lead to life (a.k.a. babies). So why does our speaker think sexual desire is fatal? Does he mean this literally? It's possible, especially given that STDs were so rampant in Shakespeare's London. Or is Shakespeare making another one of his dirty jokes? ("To die" is common Elizabethan slang for "orgasm.") Or do you think he's speaking metaphorically? So many readings, so little time, Shmoopers.