Love sickness ease... from his mistress' eye

 Sonnet 153
William Shakespeare

                                Cupid laid by his brand and fell asleep:                               
A maid of Dian’s this advantage found,
      And his love-kindling fire did quickly steep
In a cold valley-fountain of that ground;
       Which borrowed from this holy fire of Love,
A dateless lively heat, still to endure,
And grew a seething bath, which yet men prove,
Against strange maladies a sovereign cure.
But at my mistress’ eye Love’s brand new-fired,
The boy for trial needs would touch my breast;
I, sick withal, the help of bath desired,
And thither hied, a sad distempered guest,
     But found no cure, the bath for my help lies,
     Where Cupid got new fire; my mistress’ eyes.
 

This is a story about Cupid, Roman God of Love and one of Diana's maids. Cupid lays down his torch and falls asleep. While sleeping, one of Diana's maids takes advantage of his weakness and steals his "love-kindling" fire. She takes it to a "cold-valley fountain" (a spring) to extinguish it. 

However, the spring into which Diana’s maid puts cupid’s fire turns into a “seething bath”, where we can find an universal cure for illness. Any “strange maladies” are remedied by a “sovereign cure.” 

The speaker has an endless passion for the DarkLady. When he sees his mistress, or even just her eye, he becomes aroused once more. His passion is the Cupid's torch and it flares up with one look from his mistress. 

The speaker is lovesick for the DarkLady and wants to be cured from his malady. He determines that the seething bath can cure his malady - lovesickness; but it does not. He is a sad distempered guest at the bath but does not find a cure. 

The only thing that makes the speaker feel better, and his love sickness ease, is a look from his mistress’ eye.

The only thing that can cure my lovesickness is one look from his eye.