I'm in love with the concept of "ghost".
To have an eternal companion, a soulmate, not in the romantic way (or yes? go nuts, it's your life(?) but in a manner that complements and understands you; They are not just "jaja mini robots that revive us", The guardian forge a bond with the one that chooses them. In general they look for a hint of something in the cosmic lane and make choice in the moment, it is a gamble of hope and luck, to revive the right one, to be the person that they want their guardian to be, and stick together through everything.
They are supposed to revive no one else than their guardian. (yeah there is situations that contradicts that, and there are ghosts who decide to simply return to the traveler, not revive anybody or go with other people, but I believe in exceptions) ALSO, THEY ARE SO FRIEND SHAPED Anyway, thanks for coming to my ted talk.