What's he saying?
"Thine eyes I love, and they, as pitying me, / Knowing thy heart torments me with disdain,"
I love your eyes; they pity me because they know that you look on me with arrogant indifference,
"Have put on black and loving mourners be, / Looking with pretty ruth upon my pain."
Your eyes mourn for me compassionately, because they know you make me suffer.
"And truly not the morning sun of heaven / Better becomes the grey cheeks of the east,"
Neither the sun as it rises in the East in the grey dawn
"Nor that full star that ushers in the even, / Doth half that glory to the sober west,"
Nor the planet Venus at night in the west
"As those two mourning eyes become thy face: / O! let it then as well beseem thy heart"
Have as wonderful an effect as your eyes do for your face. I wish your heart would feel the same way they do!
"To mourn for me since mourning doth thee grace, / And suit thy pity like in every part."
Every part of you should pity me like your eyes do, because it would make you more beautiful to feel that way.
"Then will I swear beauty herself is black, / And all they foul that thy complexion lack."
If that happens, I will swear that only dark things are beautiful, and that anyone who isn't dark like you is ugly.
Why is he saying it?
The idea of the dark lady's eyes as autonomous mourners is a continuation of the imagery put forth in Sonnet 127, in which the poet states, "Therefore my mistress' eyes are raven black / Her eyes so suited, and they mourners seem." But in Sonnet 127, the mistress's eyes are mourning the fact that for so many others, beauty needs to be faked with make-up; she feels pity for those who are "sland'ring creation with a false esteem," or representing themselves as beautiful when in fact, they are not. However here, the pity is for the poet himself.
The theme of a poet being ignored coldly by the aloof woman for whom he writes is conventional in the Petrarchan or Sidneyan sonnet traditions. Sonnet 132 suggests that the poet wishes to be pitied since he will never be sexually satisfied by the dark lady; this theme is carried through Sonnets 135 and 136, whose main theme is sex and the speaker's lack of it. However, the following sonnets, Sonnets 133 and 134 imply that the fair lord has also been seduced by the dark lady; therefore the pity in of the mistress's eyes might be for the fact that the speaker has been betrayed or neglected.
It is also conventional of sonneteers to focus on the eyes of the woman to whom the sonnet is dedicated. They were primary in forming the connection between Laura and Petrach and Sidney and Stella. Shakespeare himself personifies them in many sonnets, often setting them at odds with the heart or mind. In The Merchant of Venice, their importance in love affairs is stated in the lyrics of a song:
Tell me where is fancy bred,
Or in the heart, or in the head?
How begot, how nourished?
It is engendered in the eyes,
With gazing fed; and fancy dies
In the cradle where it lies.
Let us all ring fancy's knell
I'll begin it,--Ding, dong, bell. (III.2.63-71)
The dark eyes of the poet's mistress are compared both to the sun and to the planet Venus. The diction used in lines 5-8 enhances this metaphor, attributing human qualities to to sky against which the sun and Venus shine. The "grey cheeks of the east" describes the sky at dawn, which is made more beautiful by the "morning sun." Likewise, the "sober west" that is beautified by the shining of Venus represents the lady's restrained and subdued manner toward the poet, made more beautiful by the love in her eyes.
Lines 10-12 make an appeal to the dark lady that her heart would feel the same way her eyes clearly do, "since mourning doth thee grace," or make you more graceful and beautiful. Line 12, "And suit thy pity like in every part," has varying interpretations because of the term "suit." One of the possible readings is the hope that every part of the lady should be "suit"ed, or dressed, like the eyes, which have "put on black" as mourners. But the word "suit" could be interpreted less literally, to instead mean that the act of pitying is clearly flattering, or "suit"ing, to the dark lady; therefore, if all of her joins the eyes in their pity, all of her will be as beautiful. "Suit" is also a pun on "soot," which means to blacken, and is used that way in Sonnet 127: "Her eyes so suited, and they mourners seem."