That thou hast her, it is not all my grief,
And yet it may be said I loved her dearly;
That she hath thee is of my wailing chief,
A loss in love that touches me more nearly.
Loving offenders, thus I will excuse ye:
Thou dost love her because thou know'st I love her,
And for my sake even so doth she abuse me,
Suff'ring my friend for my sake to approve her.
If I lose thee, my loss is my love's gain,
And, losing her, my friend hath found that loss;
Both find each other, and I lose both twain,
And both for my sake lay on me this cross.
But here's the joy: my friend and I are one,
Sweet flattery! Then she loves but me alone.

William Shakespeare

http://www.poemhunter.com/poem/sonnet-42-that-thou-hast-her-it-is-not-all-my-gr/