I think it's mainly to do with amount of handling, especially handling when they're young. My brother has a BRB that I wouldn't hesitate to hand to our grandmother, presuming she was inclined to hold it. That snake is totally docile and to my knowledge has only ever nipped my brother a few times when it was very young and still nervous.
TBH, I can't really imagine having a BRB and not handling it on a regular basis; they're so pretty, and since they hide so much, you never see them unless you pull them out. I guess that means that if I ever have one, it'll probably end up being pretty calm.![]()