I have noticed more issues with kids whose parents play and tease with them such as giving them raspberries on their tummies or arms. The kids don't understand the intensity issue nor do they have the mouth coordination to do it properly so end up grabbing on with their teeth and then the natural ooooh this feels good takes over and they bite. Does explain some of what goes on when they do it for what they think is fun as in they may truly think they are playing and not intending to hurt.

I have the same issue with parents that play wrestle with the kids. The kids do not understand that we pick them up and "slam" them down gently not for real. Once parents realize siblings especially are getting mixed signals about what is ok and what is not it actually got better.