They share one kiss, and then—
"I'm sorry," she breathes, in that ethereal voice of hers that he can't get enough of.
"This," she says, shooting him an apologetic look.
"I, Zoë Nightshade, pledge myself to the goddess Artemis. I turn my back on the company of men, accept eternal maidenhood, and join the Hunt."
Then a shower of silver light bathes her, and a girl with silver hair and a warm smile greets them.
"Welcome to the Hunt, Zoë."
And with that, his heart breaks.
He doesn't know much about himself. Only that his name is Bob, and he wants to see the stars. He finds it inexplicable. Likely, he wants to glimpse them after a lifetime in the Underworld. The Underworld is barren, with no night sky, no stars to light up his way.
So when Percy and Annabeth come looking for the Doors, he thinks he has gotten his chance. The chance to see the stars.
But slowly, he starts to remember.
He remembers that his name is not Bob, but in fact Iapetus.
He remembers her. The day she died, the day she became a constellation. A collection of stars.
He supposes that his infatuation with stars came from his infatuation with her.
And he realizes, that they are, quite literally, star-crossed lovers.
He wants to see her again. He wants to join her.
He wants to see the stars.
Perhaps that is why he told Percy and Annabeth to leave. Maybe that is why he volunteered to close the Doors. To die.
He wants to see her again.
But there is a chance, he thinks, that he will not be able to see the stars.
Percy and Annabeth can, though.
So he is not regretful when he tells Percy:
"Tell the sun and the stars hello for me."