The island has its own list

[evie]

And when the losties try to go and leave they mess with its plan. When Michael left he was happy- then something happened and he got suicidal. Hurley left and he was happy- then something happend and he went crazy. Sayid was happy then something happened and he started killing for Ben. Jack was not happy, now something is threating to make him unhappy/drunk again. Kate and Sun seem happy but both left loved ones on the island so you could say that they will never be truely happy. So is all of this a plan to get them back on the island or revenge? Walt and Aaron are the two that I'd think that the island would want back but I dont see the island making any attempts to bring them back.

[Here'sLOCKEing at you,Kid]

If we don't veer too much from the underlying theme which is determinism vs free will, I think I have only to say that the question is - just because you CAN change things, does that mean you SHOULD?... hmmm... and so the only thing clear to me seems to be that doing what the O6 did, leave the island, was something they were not supposed to do, as Jack finally comes to understand and accept. May be a butterfly effect...