The metaphor I think [i]might[/i] explain it is that of a flat car battery. If the battery is flat, you can't start the car on its own (ghost rez), but you can use another car's battery to help jump-start it (teammate rez). [spoiler]Sunsinger self-rez is like having a jumper pack in the boot/trunk[/spoiler] In other words, the Darkness is too 'intense' for a Ghost to use your Light for a revive, but a teammate can utilize their Light to help revive you. Not the best explanation, but it seems logical.