Variations in temperament

I am thoroughly amazed at the variations in temperaments of JRT's. I read all of these stories on your page about various forms of aggresion and dislike of children and then I look at my JRT "Bandit". He loves kids, well he loves everyone, especially if you want to play with him. About the worst thing he does with little kids is lick their face. I have never seen such a friendly dog. He can be eating out of his bowl and I can walk up and take it from him and he won't even blink an eye. Even when chewing his favorite bones. I have never heard him once growl at me or any other person (squirrels and rats are another item however). I have seen kids pick him up and carry him around (without my permission and before I could step in) and he could care less.

All this makes me wonder why there is so much variations in moods between JRT's. Is it strictly environment? Is it more with breeding? Will he change as he ages? Or am I just blessed?