Disclaimer: Mr. Lewis belongs to himself, his Joy belongs to his Lord, and Mr. Owen's interpretation of Jack (among many other things) belongs to Mr. Owen. I claim no rights to any of the above, I just have an idea to present which I hope might ring true with somebody else as it has with me. And perhaps it shows hubris that I try.
Inside Pandora's box, beneath the glaring Gorgon shield, there was light and hope and restoration. But there was something else, too. It was the sort of thing which could make a man ache and burn inside, and yet long to come back for more. It was an arrow to the heart much sharper than Cupid's. It was a raw glimpse into a realm to which every myth represented in the Imaginarium Geographica could only point, which gods and dragons and all keepers of the divine flame must be sworn to serve. It gnawed new wounds in Jack even as it healed the one which had been torn open at Nemo's death. He had felt snatches of such a feeling before—in a line of a book, in a painting, in the rousing chorus of a song—but this was the first time that it lingered so long. After those other snatches, he had given the feeling a name. He called the feeling Joy.
The others—they must have seen it. They must have felt it. It was playing like an orchestra, fierce and high and strong, and he was the concert hall. But when he looked up, and looked around at Bert, John, Charles, and Aven, there was none of that in their faces. Through his tears he could only see bewilderment, and traces of awe. He tried to explain the force which had touched his mind, but all he could manage was, "It's beautiful. It's full of light." And it was gone. He was shaking in the afterglow.
The light, the Joy, had come for a reason, he knew. There was a job to do. That was why he'd looked in the first place, after all—he wasn't the only one who needed healing. He got to work, praying the best he knew how as Finn's shadow ran over him, "reaffirming his belief" as Bert would say. For now, it was belief in the light, in Joy, and in whatever had made the Joy and given it to him. It was belief that the light and the Joy and the hope at the bottom of Pandora's box could heal every broken being whose soul and shadow was torn away in the Archipelago of Dreams. It was belief that somehow he would be up to the task. But later…
Later, Jack and the Joy would have unfinished business. He knew he would hunt it until he'd run it to the ground and eaten his fill. Whether in London or Oxford or the Archipelago, Jack would find the Joy's source.