Why did orpheus look back to see if eurydice is following him

Orpheus is the son of one of the Muses and a Thracian Prince. His wife, Eurydice, was stung by a viper and died after their wedding. Orpheus went to the underworld to bring his dead wife back. He stroke his lyre there in the underworld which took the madness of all creatures, the reason why Hades permitted him to take his wife back but in one condition: as he and his wife ascend to heaven, he must never look back down to his wife or he'll lose her forever.

So as they went up, Orpheus was just so afraid that Eurydice wasn't with him and he had to look back to his wife. Because he didn't believe Eurydice was behind him. And there, he lost her forever indeed. Source (s) : my senior high school notes from mythology class. : D