Psyche desires to know who her benevolent yet mysterious host is, fearing it to be a monster of some sort. Upon discovering it is in fact the god Cupid, she stumbles and awakens him. Cupid, hurt by Psyche's lack of faith in him, flees and leaves Psyche. Psyche is given three impossible trials by Aphrodite to atone for her actions, but Cupid helps her along the way.
The story tries to teach us to be faithful, but it also shows us how dangerous curiosity can be.
The Greeks valued faith a whole lot. To them, it was certainly a virtue. One must only look at how much time and resources they devoted to the many gods to see this clearly.