Analysis of Sonnets from the Portuguese iv

IF thou must love me, let it be for naught
   Except for love's sake only. Do not say,
   'I love her for her smile--her look--her way
Of speaking gently,--for a trick of thought
That falls in well with mine, and certes brought
   A sense of pleasant ease on such a day'--
   For these things in themselves, Beloved, may
Be changed, or change for thee--and love, so wrought,
May be unwrought so. Neither love me for
   Thine own dear pity's wiping my cheeks dry:
A creature might forget to weep, who bore
   Thy comfort long, and lose thy love thereby!
But love me for love's sake, that evermore
   Thou mayst love on, through love's eternity.