This is a long shot as it's too straightforward but here goes... Jack's mission was to get everyone off the island. What if everyone wasn't able to get off the island? The only 2 people we saw were Jack and Kate. Maybe the rescuers made them split up and actually only rescued some of them, while leaving the rest behind to complete the experiment.

When they were brought to safety they were told that there was an accident and everyone else died or they were told to enjoy their freedom and keep their mouths shut. Jack and Kate know that's not true but it's their word against the rescuers and since Jack doesn't seem to know how to get back to the island, the rescuers may have just dropped Jack and Kate off somewhere and disappeared.

Ben failed and was therefore replaced and banished to the real world. Jack felt that he was his only hope to get back onto the island and that's why he was crushed when he found out he died. Kate hated Ben and didn't think he could lead them back to the island and she isn't even sure the right thing to do is go back so she didn't care.

Another possible reason why Jack wants to go back is because the losties are slowly dieing one after the other. Maybe Ben convinced him that only Jacob can save them and he's the only one that's left on the island. Since he's invisible, the rescuers weren't able to find him. But this doesn't explain why Jack is tired of lying.

The weird thing is that Jack claims that he's hoping a plane crashes so that he can get back onto the island. If it was a conventional rescue crew, then they would have brought everyone to safety and reported the incident to authorities. Then it would be made public and the existence and location of the island would be public knowledge.

Since Jack is flying to different places, he doesn't seem to know the exact location of this island.

He may want to go back because life was great on the island if you think about it. Despite the peril and adrenaline rushes, there was a feeling like you're doing something worthwhile, and being needed.

Our lives here in the "real world" can't compare to that. Especially for Jack who has no one to live for really.

Yeah We aren't even sure yet if it is like that but it's like 40% that it is like that.

I think he always has to solve a problem, or "fix something."

When they were on the island he had many things to do, he felt useful, leader, etc. He had nothing to go back to in the states. Then once he got back to his unhappy life he has to find a way to get back, not only because it's a challenge, but he was happy there... in some weird way: he was.

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you know what, there is something in that last scene that keeps bugging me...
it's almost as if there was a reversal of roles...normally it's ALWAYS kate saying "we have to go back". Like, how many times has she said that in the last three seasons? Now, here she is, in the present time, off the island, and we can see that...

1. she doesn't want to have anything to do with jack
2. she doesn't want to have anything more to do with the island or
anyone else who was on it
3. she is walking away and this time it's Jack saying "we have to go back" and she just leaves

something is not right, and I think there's more to it than just them realizing their lives off the island are not as meaningful as they were on the island. Locke keeps saying stuff about "this isn't the way it's supposed to happen" or "jack you weren't supposed to do that"; there is a destiny involved with what they do and the consequences are far reaching, probably affecting time itself...