I found the following from Till We Have Faces, by C.S. Lewis:
"No, no, no," she said. "You don't understand. Not that kind of longing. It was when I was happiest that I longed most. It was on happy days when we were up there on the hills, the three of us, with the wind and the sunshine... where you couldn't see Glome or the palace. Do you remember? The colour and the smell and looking across at the Grey Mountains in the distance? And because it was so beautiful, it set me longing, always longing. Somewhere else there must be more of it. Everything seemed to be saying, Psyche, come! But I couldn't (not yet) come and I didn't know where I was to come to. It almost hurt me. I felt like a bird in a cage when the other birds of its kind are flying home."