I believe it's a reflection of the household they grow up in.
I've never had to use physical force to discipline either of my kids and we constantly get compliments from others when we're out on how well behaved they are.
We don't scream, we don't hit each other, and we recognize the need to treat each other with dignity and honesty.
Some of their friends, though, the home is a hurricane of chaos, and those kids probably do need corporal punishment.
Nope. I had one that was, um, strong-willed. At a very young age. He now has 2 just like him. I laugh. :-)