Urvashi and Pururva lived sixty thousand years together. Before coming with him, she put forth three conditions, violating which she would leave him. These three conditions were:
1. Two sheep would always remain close to her bed. If by any chance , the sheep is not there, she would return to heaven.
2. She would never see the king naked.
3. She would be served ghee as her food.
Gandharvas planned to have her go back to heaven. The whole story is here: