our lovely lab doesn't nip us (or very rarely) but always goes to nip if a visitor or someone out on a walk puts their hand down to stroke her. Of course we say 'no' firmly and ignore (sometimes leaving the room/putting her in her crate if too bad) but by then the damage has been done and nervous visitors/small children are scared to death! Or if we are out on a walk, I am so embarrassed!
She broke the skin on my dad's hand the other dad and that is clearly not on. She is very excitable generally with strangers/visitors, always has been.