Alright, so considering the fact that Jack died a horrible icy death, this argument is entirely academic. But had both Jack and Rose survived, had they been on the Carpathia instead of the Titanic, their relationship still would have been doomed. Everything you see between the two characters on the ship is a classic example of the beginning of a relationship between two very young people. Rose may think that Jack's nomadic lifestyle is exciting and romantic, but at the end of the day, she's a rich girl. She's gotten accustomed to a certain way of life, and while she might not enjoy the hypocrisy and snobbishness of her fellow one percenters, she's probably going to be singing a different tune the first time she's asked to sleep on the street. And Jack's aimlessness will start to get a lot less charming. As for Jack, if he's the kind of guy who wants to be able to pick up and leave at a moments notice, a long term relationship has to feel pretty inhibiting.