LOST (spoilers for aired S6 episodes!)

[Mervi]

Why does the afterlife (Or purgatory, I guess I should say) take the form of a life in which they never crashed? Does this kind of thing happen to everyone or just the people who were on the island?

I think the explanation from Jack's dad was that the afterlife (or "inbetween" if I may borrow a term from the Lovely Bones) was what their minds created together, because these were the most important people in their entire lives. So, in a sense, other flashsideways/inbetween worlds might exist for other people (or maybe they all exist together, after all, "there's no here, now") but this what they as a group imagined. And the reason they never crashed in that world is that they are all trying to "fix" things that went wrong in their lives, but they need to learn to let go of all the pain: the plane that crashed, the relationships gone wrong etc. That's why Jack has a son he never "really" had - he wants to prove to himself that he would have been a better father than his own father was. Ben wants to fix his relationship with Danielle and Alex, and to be a better person at work as well. Hurley wants to do good things and be liked even though he's a millionaire etc.

Well, that's one interpretation of it.