During the first seasons, the show made a great deal of Jack slowly becoming (unwillingly, at first... even if he became an asshole (an incompetent one, too) on that front later on) the leader of the crash survivors. After that though? Not a word about what had happened to 90% of the group while he was back on mainland. Does he even know? Does he even care? Did he even ask? After all, he didn't seem interested in what had happened to them over the last three years, when he was reunited with Locke in that hospital. Considering how much trouble they'd been through in merely a few months, that's incredibly nonchalant.
His sudden and character-altering off-island depression was never explained. Like said above, shortly before the Ajira flight, Jack pretty much told Locke to go fuck himself and apparently wasn't interested in knowing what had happened to the rest of the group. But then, according to the timeline, he breaks down, takes pills, drinks, etc, until "we have to go baaaaack!" Why? Never said. The only detail that apparently shook Jack in that hospital scene was that Locke claimed he had seen Christian. Guess Jack didn't remember seeing his father there as well, and even discussing that with Locke afterwards. Season 1, man. Can't blame him (or the writers) for forgetting, I guess!
Season 5 tells us he's fine with detonating a hydrogen bomb because of what happened between him and Kate. Okay.
(Of course, that plan also means that he most probably wouldn't get another chance with Kate, considering how she was in shackles for murder one, but hey.)
Why is Kate back, by the way? Oh, right, she suddenly realized that Aaron wasn't her son. So she wants to find Claire (never mind how she also told Locke to go fuck himself shortly before coming back... see, the writers just wanted Locke to be suicidal by the end of the episode, with those scenes, consistency be damned... and yes, we'll still pretend in the end that the entire show was about how all those characters cared so much for each other, why do you ask?).
Funny, then, how Jack doesn't think of telling Kate about how Claire supposedly turned evil. You'd think that might be helpful. You'd think that's one thing he'd care a bit for, if he's (literally) crazy about her. But no, that Claire revelation was just supposed to be an end-of-episode "damn right we're bringing back a character whose actress had been missing in action for a while! how awesome are we?!" moment, nothing more.
And in season 6, after smashing a magic mirror because, Jack now has faith in the plot.
"That Jacob guy I just met yesterday, who put us all in danger for years and didn't bother to explain why until most of us died (and even then...) seems like the kind of guy I can blindly trust! Surely, he wouldn't have brought Desmond back for no reason! He has to be a weapon against the Man in Black, even if it would make no sense at all for Jacob not to tell us about that last night when that exact issue was brought up!"
Turns out he was right. Jack can read the Matrix, plot holes and all.
In the end, Jack, the guy whose character flaw had been said to be that he always had to try and fix things, sacrifices himself to fix the island. And then, after his death, he's still all up in Locke's shit because "gotta fix stuff!" The character has grown so much.
But thankfully, the limbo stuff wasn't all pointless: it showed us that Jack would be a good father. So that solved his own daddy issues. Obviously. And I know I can't get enough of the show telling me how "awesome" Jack is, so that's good, too. Even if said son turns out to be fake and is never mentioned again after that. His work was done.
Oh, and he ends up with (the absolutely fantastic character that is) Kate. That's really important.