Patty wants to know if scarecrows work on squirrels.
But this leads to him becoming immortal, and once Jack can't die, he also can't die for any one else, not in a way that means anything, not in a way that speaks to the little boy who maybe didn't want to be a soldier, but sure did want to be a hero. It's a terrible thing, and one that robs Jack (and the audience) of a lot of illusions -- we get hints at this through Torchwood seasons 1 and 2, but it's only in Children of Earth where we really get the message: being a hero is not romantic; war is not romantic -- and I think that's so important, because woe, Jack does make being a hero look romantic, does make war look romantic, and those are terrible things to think, ever.
When Ianto dies, Jack has nothing to bargain with, because his life is not his own, even as he will live it endlessly. When he uses Stephen to defeat the 4-5-6, it is the same -- he cannot act as surrogate. Jack cannot take another's pain onto himself to save them, rather he can only live with pain unimaginable to any of us.
If we look merely at where CoE ends, the message is bleak and pretty horrible. I certainly walked out of my first viewing of it feeling destroyed for Jack and being angry that he was, I thought, a coward. But without his own life to give, it's not that Jack's a coward, it's that he can't be a hero in the simplistic lexicon we all know and all think we'd like to play with and would engage in properly if push ever came to shove in our own lives.
However, if we look at Jack's arc as it relates also to Doctor Who, specifically Gridlock, it's a particularly unique Hero's Journey. Jack -- the eternally youthful and exuberant boy -- must go on alone, for whatever reason, for whatever trials until he is alone enough with himself (in his identity as the Face of Boe) until he can once again give up his own life for millions. And there, in that moment, in Gridlock it's beautiful, not because Jack gets to be our childlike notion of a hero again, but because it is him being returned, for just a moment before he goes, to the life he once had -- one about which he actually has the power of choice.
In the end, we have no idea how many people Jack has loved that he has had to sacrifice directly or indirectly because he loved them. But in the end we do know that Jack -- who really does have epic amounts of self-loathing -- ego aside, finally loved himself.
So yeah. That's my story, and I'm sticking to it. (and I'm also thinking.... hrrr, there's some academic analysis to be done somewhere close to this on this....)