What is the lesson of Orpheus and Eurydice?

The lesson in Orpheus and Eurydice is the importance of trust. Orpheus has to trust that Eurydice is behind him as he travels away from Hades because he is forbidden to turn around and look at her until they have left the underworld.
Though it seems easy, this is actually a very difficult task for Orpheus. After all, he already feels betrayed, as death took Eurydice on their wedding night. How bad is that? Wouldn't that fill anyone with fear that the universe is against them? Orpheus had to be very worried that death would trick and cheat him once again on the threshold of happiness. He must have felt he had to give just one little tiny glance back to make sure his beloved was going to truly follow him out into life.
Of course, this lack of trust was his undoing. He needed to trust the gods of the underworld and Eurydice. While none of us will be faced with having to go literally to the underworld to retrieve a dead spouse, the story holds a universal truth: at times, all relationships face being 'poisoned' by something similar to the snake bite that killed Eurydice—maybe an affair or financial irresponsibility. At that point, rebuilding the relationship, which can seem as if it has died, can feel like a trip into hell. But to rebuild, reestablishing trust is important.
The lesson is that we have to trust the universe and our loved ones. We can't keep looking over our shoulders—even once—and second-guessing.