I think Jack was a mess before he crashed. He couldn't move past an unsuccessful relationship, couldn't deal with his daddy issues. Yes, he was strong, and compassionate as a doctor but his flaws were undoing his potential. The island removed all that baggage, his strengths were allowed to grow stronger. He flourished in the rebirth that the island offered. All the losties had the same opportunity, just as Locke once told Charlie I think.....this is our chance to start over clean. Once Jack goes back to the real world, he mourns the loss of his island family and the best part of himself.......then he becomes his own father.

To answer the question about Jacks future staying on the island.... I think he would have opportunity to grow and change for the positive. (redemption, getting rid of baggage) He has a strong functioning family that gives him what he needs. He is fulfilled as a person.