A friend of mine came to me after watching seasons 1-5 online at breakneck speed (about 2 months!) with what he thought was the flaw of flaws in all LOST logic: Jack and the rest of the O6 came back to the island to "save everyone who they left behind" however, unbeknown to them, and like Juliet said in 'Whatever Happened, Happened,' they "didn't need saving." While I merely replied that this wasn't so much a plot hole as it was a manipulation on Ben's part and somewhat an ignorance on Jack's part to assume that Sawyer, Juliet, Jin, Miles etc were in a great deal of danger, my friend was really irritated that the O6's return to the island was ultimately unnecessary (to which many of us would reply that they never should have left in the first place and the island is where their destinies lie). I still thought about this a great deal after we had this conversation and I figured Jack still thought he had to save "everyone" and his solution to this was to detonate Jughead. However, I don't think Jughead was the right course of action to take (maybe because I find that blowing up a hydrogen bomb so that Faraday can have another shot at Charlotte and so Jack can have another shot at Kate is bogus) even though Jack does still need to save someone. Notice I wrote someone not everyone. That one person is John Locke. Locke was so adamant about Jack not leaving the island and Jack knows that his death is ultimately his fault -- and we all know Mr. Fix-It can't just let things be -- so what I'm trying to say is I think Jack's true motivation for going back, for blowing up Jughead, isn't really so he can get another shot at Kate, it's so the Man of Faith gets another shot at life. Maybe I'm giving Jack too much credit and he really just is that in love with Freckles, but I'm one who thinks that the final shot of the show will be reminiscent of Jacob and the MIB's beach conversation from 'The Incident' -- only with Locke and Jack sitting in their positions. I don't really have a theory on how exactly that's going to happen (loop? more time travel? agelessness?), I just tend to think that Jack believes that if he can save Locke then everything else will work out okay.
All in all, I just hope that Jack was trying to save the black stone to his white (or visa versa) and not just right the wrongs of his love life.