Oh jesus brain, thank you for reminding me that Jack talks about going to war when he was a ‘boy’ and that the attack on Boeshane probably happened when he was around ~13 (I believe the actor who played young Jack was 14? someone correct me if I’m wrong) and how *Jack lost his childhood in a matter of minutes* and how painfully perfect the details we get in some of the novels that he really enjoys arcade games as stress relief etc are because of that.
I tend to feel like Jack acts like he does because he doesn’t want to get too close to anyone in general, and he was that way even before he became immortal. If what he said in “Captain Jack Harkness” was true, besides losing his brother he also lost his best friend. And feels guilty for his death as well as Grays.
And that’s where the Doctor comes in, and Rose. Because he grew to care about them despite himself. He couldn’t be amoral and disinterested like Hart, and I wonder if that’s part of why the Time Agency took those years from him.
And now, of course, he can’t help losing people. Even if they live for a time, they always leave. Or Jack leaves them. So he smiles and flirts and plays because it lets him run away emotionally, at least for a little while.