I understand the the Prime Directive strictly prohibits many things in regards to interference with other less technologically advanced cultures through artificial means. However, why does it seem to be taboo for a Federation citizen who just wants to live primitively to live with such a culture? As an example I am using episode 13 from Season 7 of The Next Generation (Homeward) where Worf's brother decided to stay with the culture he saved instead of returning to the Federation. Maybe it is due to his knowledge of processes, technologies, ways of doing things, but if he just wanted to live without all the advancements, I would not see this as a problem. Worf immediately mentioned that he could not stay but gave no specific reason as to why. Is it the Prime Directive? Or something else? Any thoughts?
Nikolai Rozhenko, as a Federation anthropologist is bound by the Prime Directive. One thing to keep in focus is that their (the Boraalan's) entire living culture and every last person in it would be dead if not for his actions. If not for his interference they would have 'naturally' ceased to exist. I can understand his compassion. It seems noble and right to some, probably to many. But.
Now they're alive on another planet that doesn't biologically 'know them' and interacting in ways that could never have been, but for his interference. Will their unintended presence there unbalance or damage that entire planet. Will their culture now evolve and take a whole different direction? Will they eventually become a threat to other planets? Did they bring their 'natural' virus' to this new planet's indigenous life forms that have no immunity to them?
It's easy to say he was being compassionate to save them but in the process could set forces in motion that far outweigh the numbers he stopped from dying a natural death. Often unintended consequences are far more damaging and lasting than intended consequences are. Hence the Prime Directive.
Sooner or later someone will probably notice that Nikolai Rozhenko is not really Boraalan. That alone could pollute a primitive culture let alone the damage that what he says or does could do. For example, consider the possible side effects of Riker ending up in a hospital on Malcor III in First Contact (the episode, not the film); all of a sudden various people know that there are aliens: they've seen them, they've seen the x-rays, they've even "known" them in the biblical sense. Lucky for Malcor III, the alien could be dismissed as National Enquirer style tabloid nonsense. A less advanced culture would probably react with superstition as the Mintakans did in Who Watches The Watchers.
As far as seemingly innocent things that Nikolai might do, consider James T. Kirk in The Paradise Syndrome:
As they are speaking, a boy who has been drowned was brought in. Salish states that "there is no sound in the boy, there is no life in the eyes, he will move no more." As he moves off, defeated, Kirk moves in and tries CPR on the boy. Within a few moments, the boy is breathing again.
Kirk has inadvertantly influenced the local "primitive" culture by showing them advanced first aid techniques (technically no CPR was involved, just some leg pumping and mouth-to-mouth, see the discussion in the comments).
Just the presence of an alien (such as a human) from an advanced culture (such as The Federation) can interfere with the development of the local civilization. The Prime Directive is meant to avoid interference, the presence of an alien can violate the principle behind the Prime Directive.
Other answers point to the Prime Directive as the answer, which is a valid conclusion, but I don't think it's necessarily the case here (or at least not the only factor). Nikolai was violating the Prime Directive throughout the entire episode, and Worf made it clear that he thought this was a Very Bad Idea. But by the end of the episode, which is what you're talking about, the major opportunities for negatively affecting the Boraalans have all passed. And he already has one of them pregnant, so the damage is done (I could see cross-species interbreeding, when the native species doesn't even know other species exist, as a potential problem if she weren't already pregnant; but by this time Worf knows she is.)
So I don't think the Prime Directive is Worf's main reason for telling Nikolai he can't stay. After all, when he was studying them the Federation allowed him to live there for a long period of time. Presumably he wasn't supposed to enter into an intimate relationship with a Boraalan, but just living there in disguise wasn't a problem. It seems that all the opportunities for Nikolai's presence there to make things worse have elapsed, and that any that would arise in the future wouldn't be any more likely than if he were still saying there as part of his observation mission.
So I think the real reason Worf told him he couldn't stay is because it's unlikely anyone will ever come back for him, and Worf didn't want to never see his brother again. It's highly unlikely Nikolai will ever see their parents again. Worf didn't want that for his family, but in the end he realizes Nikolai will be happy and so he leaves him there. (Also if it were really in violation of the Prime Directive for him to stay, Picard wouldn't have let him. So while I understand the logic, I really don't think that's the answer.) I think this is purely an emotional response on Worf's part; he's not saying that Nikolai actually can't stay (remember, he did stay!), but that Worf doesn't want him to stay.