Interstellar travel
One reason why very long (generational) interstellar trips are a favorite SF theme, is because they never succeed the way they were first planned, thus making great stories.
People living in a confined volume all their life , live in a jail, live like within a sect with a necessary authoritarian command.
It is so unlike human's desire for freedom. That's also why it is a SF theme.
Those people are running away from an undesirable world but I'm afraid they won't like the world they want to reach for the same reason they left Earth. The planet is not the problem, it's what's in their mind.
Still a great SF theme. N'est-ce pas, Socrate II;-)