Believe it or not, humans can and do live in isolation in single family units all over the world. It's not exactly a good environment to grow up in, but it doesn't turn someone into a murderous beast or whatnot. Do also realize the alternative. Every person you add - and their corresponding mass overhead - dramatically slows down your spacecraft or dramatically scales up the size of its launch stack. There are huge costs; launching only the absolute bare minimum is critical. The slower you go, the more consumables you need, which increases its mass on its own (see the aforementioned problem), and the greater the likelihood of catastropic collapse).
No, a semi-working society ready to hit the ground is most definitely not a requirement, you don't want a "society" until you have a base set up that can sustain them, and automatic robotic labor, not human power, is far more efficient (both in mass and energy terms). But even if such a ready society was a requirement, that'd only be a task for the last generation or two, not every generation in transit. Again, minimizing mass (and thus in-transit consumables) is critical.
The biggest challenge of the whole thing is the massive difficulty of setting up a truly independent colony with as little hardware as possible. Modern technology is based on ridiculously long resource and production chains, with each part produced requiring consumables from numerous other different production chains; a single random piece of modern technology may ultimately have required some tiny degree of the usage of tens to hundreds of thousands or more chemicals or parts that will eventually wear out, in thousands of systems scattered all across the world. Every bolt on every mining truck, every hydraulic fluid in every piston, every seal, every fluxing agent, every tire tread, every hammer-mill hammer, they all need to able to be produced at a faster rate than they are consumed. One obviously gravitates toward "generalized"production processes like 3d printing for part manufacture and plasma centrifuging for refining; however, low volume / high consumable production techs such as these may be fine for low volume consumables but become a consumes-more-than-it-yields dead-end when it comes to making high volume consumables. And you can't just send an emergency resupply boat to a different starsystem. It's fundamentally critical that technology chains be condensed to their absolute simplest cores. Nor are all of your raw mineral inputs going to be found in the same location, which means a cross-planetary shipping system is needed. All of this sort of stuff is the real challenge.