Calypso's never loved anyone as much as she loves you.
That's why she has to let you go. It's unfair to keep you on this island with her, as much as she'd love you for company.
But she wants you to. Deep down, she wants you to be the one to stay, to choose her over the rest of the world.
But that's not how things work. Not for her.
You've been on Ogygia for a week, (or at least you think so,) and you've grown close with Calypso very fast. She trusts you. You're kind to her, and you listen, and you enjoy being around her. That's all so new, somehow.
You're both hanging out in her cave, and she's weaving a sweater for you as she listens to you talk about your life. She nods along, asking questions here and there, and seeming pretty engaged.
Then you mention your friend Annabeth, and she stiffens. "...Who?" Calypso asks in a small voice, not making eye contact with you.