When Eurydice, the woman he loves, dies, Orpheus is so grief-stricken that he decides to travel to the land of the dead, also known as the underworld, to bring her back. Nobody has ever returned from the land of the dead before, and so this would doubtless be a difficult and dangerous journey. In the land of the dead, Orpheus finds his way to the king and the queen of the underworld, Hades and Persephone. He tells them that Eurydice has been taken too soon, and he asks them to send her back to the land of the living. Hades and Persephone were so moved by Orpheus's plea that they agree—but on one condition. Eurydice can leave with Orpheus, following behind him, but if Orpheus ever looks behind him to check that Eurydice is really there, then she will have to remain forever in the land of the dead.
This one condition may seem strange and rather arbitrary, but it is arguably a minor condition, given that Eurydice will be granted a return to the land of the living. Orpheus, however, is unable to fulfill this one condition. As he is about to leave the underworld, he looks back to check that Eurydice is behind him. As soon as he looks back, Eurydice is doomed to remain forever in the underworld. One can see why Orpheus gives in to temptation as he does, but the fact that Eurydice has to remain in the underworld is arguably his fault and not the fault of Hades or Persephone.
I think if I were a god or goddess, I wouldn't handle this situation differently. It seems fair for Orpheus to be tested in some way in order to deserve what he is asking for. I think the test that Hades and Persephone devise for Orpheus is not unreasonable or unfair, given what they are willing to grant Orpheus in return. It is, after all, quite unprecedented for a deceased person to be granted another opportunity at life.