Our guy comes from good lines and we have been vigilant in socializing him, with both dogs and people. However, since the day we brought him home until now, he has had no interest in being pet or cuddled by strangers. Funny coming from a breed that's very people oriented. He'll tolerate it if treats are being given at the same time, but given the choice, he moves away from them. Our family can give him as much loving and cuddles as we like and he'll take it all in happily - but strangers, or even neighbours that he's met dozens and dozens of times, he has no love for. There's no growling or anything, he just moves as far away as he can with most, or will randomly decide that 'this one's ok' and go up to them himself. It seems to be a crap shoot. Both of his parents are typical Newfs wanting to be loved on by all, and I bristle if someone were to suggest that his is a result of 'nurture', as I think he's out and about and in training more than most dogs - he just loves his own people and has no need for others. Dogs on the other hand, he adores and will do anything to play with them - even if he's getting snapped at by a yappy little thing, he stands there, tail wagging.
So while I think there may be an element of breeding in some cases, and it may be nurture in some others, I think there are just different personalities in some dogs, Newf or not, that lead them to act in varying ways.
Edit - Jager's mom mentioned "Why does it seem like the breeding of these dogs is going the wrong direction?" - I wonder if this is actually true or if it's like many things now a days and it's not in fact a problem that's becoming worse, it's just one we're hearing about more now that there are so many technological ways of connecting with others. Crime in Toronto is a great example to use for this - every year, the majority of people think that it's increasing, as there are terrible stories on the news and in the paper every day. But the actual truth is that it's declining - it's just being reported more often, thus giving us a skewed perspective. I wonder if there hasn't always been a wide variety of Newf personalities, but we lacked forums such as these to connect with other owners and hear about them.