Jack's grace reaches for someone to anchor himself to. It should be Lucifer, but the bond he has with his father waxes and wanes, a string twisted around the pillars of other universes and pulled thin.
More importantly, Jack is a child, and he needs someone here. To talk to. To hug.
Sam is kind, gentle, believes that Jack is good. His soul shines like Jack imagines Lucifer does.
Jack only has to wait. Lucifer will come back for them both, nephil borne of his grace and human whose cells still glow with his light. They will be a family.