This is going to be one of those posts that are incomprehensible to anyone who isn't versed in the ways of the new Doctor Who, plus Torchwood, and there's just nothing that can be done about that.
I've always noticed a difference in the way Captain Jack is portrayed on Doctor Who versus the spinoff show, and last night, mid-episode, a perfectly sound explanation for it popped into my head.
When he first appeared on Doctor Who, sexiness was pretty much Jack's defining characteristic. Then when he came back on his own show, he was much more in the serious, broody genre hero mode. Unless I'm totally forgetting something (which is quite possible at this hour), I think he's the only person on the show who hasn't gotten laid so far, and that just seems WRONG.
Then, when he guest-starred back on Doctor Who's third season, even with a more serious side in evidence, we still got to see more of the old Jack again, and almost immediately (for example, the exchange that went something like, Jack flirtatiously asking "Can't I even say hello?" and the Doctor saying "NO!")
The seeming change in his personality actually seems plausible, though. On Torchwood, among ordinary human beings (albeit smart ones, who've all had alien encounters), Jack is unusually long-lived, and has had experiences that are beyond their understanding. He's been to outer space, he's lived in all sorts of time periods and knows all kinds of things that others do not (plus, of course, he can't die), which gives him a burden, and also isolates him to an extent.
But from a perspective that includes the Doctor, all these things that are so unusual on present-day Earth are no big deal. Next to the Doctor's enormous life span and huge range of knowledge, Jack's just an ordinary guy.
Also, among normal 21st-century folks at Torchwood, he has responsibility. The buck stops with Captain Jack. As soon as anybody's in the periphery of the Doctor, well, the buck's always going to stop with the Doctor, no question about it.
So once Jack's around the Doctor, it frees him up to be his lighter, frothier, flirtier self again. Which is actually the Captain Jack I tend to prefer. Does that make me shallow?