There are a number of variations on the myth of "Orpheus and Eurydice." For quotes, I will focus on one of the most widely-accepted versions, which is in "Book the Tenth" of Ovid's Metamorphoses.
Long I my loss endeavour'd to sustain,
And strongly strove, but strove, alas, in vain:
At length I yielded, won by mighty love;
Well known is that omnipotence above!
This quote focuses on Orpheus's reaction to the loss of his beloved Eurydice, who died after stepping on a viper early in their marriage. Here, Orpheus is talking to Hades and Persephone in the Underworld. He has traveled to the Underworld to bring Eurydice back and is trying to explain his respect for tradition and universal laws.
Hades is charged with keeping the spirits of the dead in the Underworld. He is a very just god—meaning that he does not make exceptions to the rule that, once people die, they stay in the Underworld for eternity (as exceptions would not be fair to all the other souls). Orpheus is explaining that he understands that he should have moved on with his life after Eurydice died, but he was unable to. This is a good strategy to use with Hades, as the god himself knows the power of love, having kidnapped a woman and forced her to be his wife because he fell in love with her.
But if the destinies refuse my vow,
And no remission of her doom allow;
Know, I'm determin'd to return no more;
So both retain, or both to life restore.
Here, Orpheus is demonstrating the same power of love that initially compelled him to journey into the Underworld to retrieve Eurydice. He is imploring Hades to release her but also saying that if Hades will not allow Eurydice to leave then Orpheus will stay in the Underworld with her. In other words, "either let her come with me or I am staying here with her." Orpheus's words reveal the depths to which he is willing to go for his love, even going so far as to sacrifice the remainder of his earthly life to never be parted from her again.
They well-nigh now had pass'd the bounds of night,
And just approach'd the margin of the light,
When he, mistrusting lest her steps might stray,
And gladsome of the glympse of dawning day,
His longing eyes, impatient, backward cast
To catch a lover's look, but look'd his last;
For, instant dying, she again descends,
While he to empty air his arms extends.
Again she dy'd, nor yet her lord reprov'd;
What could she say, but that too well he lov'd?
This quote depicts the journey of Orpheus and Eurydice from the Underworld back to the surface. Hades has relented and allowed Eurydice to leave, under the condition that Orpheus not look back at her until the two lovers reach the surface.
Unfortunately Orpheus cannot bring himself to trust Hades's word, and just as they are reaching the surface, he looks back. Eurydice is immediately pulled back into the Underworld, and Orpheus is unable to re-enter to try to get her. Perhaps the most powerful part of the quote is in the two final lines. As Eurydice dies again, she bear no anger toward her husband. She understands that, just as it was love that compelled him to journey into the Underworld to get her, it is love that compelled him to look back and make sure she was there. It is a very bittersweet, tragic moment.