Disclaimer: I don't own Torchwood and I am not making any profit from this work.
Ultimately, Jack is alone. Although he has friends and colleagues, and he goes home with Ianto sometimes, and he still talks to Tosh and Owen in the morgue, and he looked out for Estelle long after their affair was over, he is, in the end, alone.
Jack's curse is that he will be left behind. Most of them have been left behind by somebody. By the end of a life, most people are. Gwen and Tosh had been left behind by grandparents, elderly relatives, the odd family or personal friend. The unluckier, like Owen and Ianto, had been left behind by those they had not expected to lose, and certainly not so early.
But Jack will be left behind by everybody.
Unless Jack is 'fixed', he will outlive even the Doctor. He will outlive everyone and everything except time itself, and time is hardly a good companion. Time can't talk and laugh and joke around, and most of all, it can't make things feel better. Time can't heal when its passage only means that you are going to lose more and more and more, until nobody's left.
When Jack stands on his rooftops, he feels alone, but he can still see that things are alive and well and, importantly, they're carrying on. And he knows that they will carry on, in one way or another, for several more centuries. But Jack also knows that there is an end of the world, for humanity, and that he will have to outlive it.
But worst of all, Jack can't distance himself. He can't keep himself locked out, like Owen does, or detach himself entirely, like Ianto can. He can't stop feeling and caring and loving this silly, messed-up species. He can't stop wanting them to be okay. So every time, he falls in and out of love, and gets his heart broken, and feels anger, hatred, love, affection, longing, joy, contentment, misery, despair - everything that makes him and everyone else around human.
Jack has two curses, really. The first of immortality, and the second of having a heart at all.
It keeps him painfully alone, and horribly aware of the fact.
Gwen didn't come to Torchwood alone, but that's how she will end up. She came with her loving boyfriend, and her friends from her old job, and her family in the background, and her social buddies from old schools and old hobbies. But Torchwood will isolate her, and mould her into the same stuff that has made the others so quietly, fiercely tragic.
Her relationship with Rhys is fractured and tenuous. The loyal, loving Gwen was provoked into an affair with a man who wasn't looking for anything more than a fling in the first place. The blue-eyed girl who was besotted with her boyfriend and wouldn't look at any other man is suddenly eyeing up her boss and wondering if it would be impolite to ask Ianto what being kissed by Jack, like he loves you, is really like. The naive, innocent young woman who thought the world was literally as it seemed is learning things that set her apart and isolate her, until she can only seek refuge within Torchwood itself.
For the moment, she hides and tears her loyalty and resists, because she knows it's happening. They all know - they, perhaps, more than she does. They've seen it before and, if they live long enough, they'll see it again.
So while Gwen, unlike the others, can go home and have a relationship and cook and clean and watch TV and laugh with a lover and chatter to her friends, and confide in someone, it won't last. Because her job and the secrets and the wonder it involves are nibbling away, crunching and munching, until the ordinary bloke that Rhys is and always will be...just isn't enough any more.
Gwen is destined to be alone, but the pain will come from the fact that she has to lose what she has to get there.
Owen kept himself alone, and it's unsurprising.
After losing Katie, and the painful way he found Torchwood, the organisation really didn't have to do much into keeping him alone. Owen did it all for himself. He shut himself off, hiding behind his spiked barbs and his callous insults and his general asshole demeanour.
It was as effective as putting headphones on and blasting the music loud enough for the entire train to hear. His attitude kept the relationship between himself and Ianto firmly on the 'amused acquiantance' side of things. They never would have been friends. They never would have met up outside of work, or gone to see a match together, or got drunk together and swapped lurid stories of girlfriends in university and school and what the best lay they ever had was. They were colleagues, with a bit of banter, and not much else.
It was Owen's attitude that kept Gwen at arm's length, even with their fling. His attitude that kept it from becoming long-term, and kept her from finding out anything new about Owen from sleeping with him aside from what noises he made in bed and what he looked like under the lab coat and crappy jeans.
And most of all, it was Owen's attitude that kept him away from Tosh. He couldn't let her in, and couldn't ease the loneliness that they were both constricted with. He couldn't do that, for whatever reason, and they remained isolated, despite their attraction.
But once burnt, twice shy, was a rule that applied very well to Owen, and by the time he realised that not everything that looks like fire necessarily is, it was too late.
Owen was alone, from the day he learned of aliens, to the day that he died. But at least for Owen, it was largely done by himself.
Tosh was perhaps simpler than any of the others. It seemed that Tosh had always just had a run of very bad luck. Wherever she had found companionship, it had turned out badly.
Tommy had been doomed from the start, and they all knew it. At least there, Tosh knew too, but since when did the head rule the heart when it came to being in love? Whether or not she'd truly loved Tommy, she had liked him, and found him a good companion. Being forced to give him up was a difficult day, and it had hurt, no matter how much she had tried to hide that.
Mary had been the unmitigated disaster. Tosh had fallen, in one way or another, very hard for her. And losing that, whatever it had been, had been devestating. Like Owen, she had withdrawn after being burned, even though she craved companionship. She craved someone to love and who would love her, and it had made Tosh into...a tragedy.
It was love, essentially, that drove Tosh into her loneliness. What she loved was destroyed and turned away. What she merely liked was not. She had, briefly, found friendship with Ianto, together in their loneliness, but then her time had come and it was too late to learn from her mistakes and try again.
Maybe, had she lived, Tosh would have been the exception. Maybe she would have found love, and she wouldn't have been alone anymore, and she would have broken the terrible trend that Torchwood kept hanging over their heads. But then, maybe that was why Torchwood killed her.
Ianto, it seemed, had always been alone. He rarely spoke of his family, rarely spoke of his friends, and spent so long at work that he couldn't have regular contacts away from Torchwood itself. He hadn't mourned the dead at Torchwood One, and didn't seem to have lost anyone but Lisa.
Lisa had been Ianto's breaking point. With Lisa, even damaged and broken and not-Lisa anymore, he had a tiny part of his heart that wasn't completely alone. But when she had died, that had shut down and died as well. Now, Ianto was alone, and worst of all, it seemed to fit, as if that was his normal state, and not the happier, softer, tender young man that had cared for Lisa so diligently.
Ianto had loved her desperately, and rightly so. She had kept away that crushing loneliness that Torchwood didn't have to give him, because he already knew it, inside and outside. And her loss had brought it screaming in to occupy his every waking moment.
It sits behind his eyes at every turn. A wall stands between him and the rest of the world, and even when he does seem to connect, in the dark nights in Jack's room below the office, there is something distant and strange about him. Ianto doesn't belong with them, or with anybody. Ianto is a man who is designed to be alone, and was never meant to realise how terrible a fate it was.
But Lisa took him out of the loneliness, and he depended on her to keep him out of it. Only she couldn't.
And Torchwood couldn't - wouldn't - bring him company again. That wasn't what Torchwood did. Torchwood set you apart and isolated you, kept you alone and singular and cool, until the day you died.
But Ianto was already there.
Torchwood is loneliness. Together, they are alone. Torchwood destroys its members while it saves the world, and they will never be heralded as heroes because they, to the world, will never exist.
They are lonely shadows in the background, and they will never register. Not really.