I believe we lose immortality because we have not conquered our opposition to death; we keep insisting on the primary, rudimentary idea: that the whole body should be kept alive. We should seek to preserve only the part that has to do with consciousness.
And I don’t see how Sebastian can be all that fond of Jace, either. He was horribly jealous of him all his life. He thought Jace was Valentine’s favorite,” added Clary. “Not to mention,” Magnus noted, “that Jace killed him. That would put anyone off.