Thine eyes I love, and they, as pitying me,
Knowing thy heart torments me with disdain,
Have put on black and loving mourners be,
Looking with pretty ruth upon my pain.
And truly not the morning sun of heaven
Better becomes the grey cheeks of the east,
Nor that full star that ushers in the even,
The noun “ruth” is archaic, a form of the verb “to rue” meaning “regret” or “pity”.  The adjective “pretty” is ironic; a more serious view would normally be more appropriate. 
 Alliterative ,  plosive  “p"s in line four may suggest exasperation or sarcasm. The quatrain is full of tension and accusation.
Sonnet 132