Have you ever seen a growl and a grrr and a roll and a totally wrestlemania rough-house experience with your dogs?
We had guests over that assumed they were going to kill each other but they were having fun.
It's important to distinguish between play and aggression. There is a huge difference but perhaps not to a person that hasn't seen both in action.
Puppies play as if they are going to gnaw each others tail, hair, ears or what have you off but they know how hard to bite down and if one was to go yelp, the other would stop and then continue and lessen the impact. They don't want to end the play and if they don't 'get' it, well they won't have anyone to play with.
If you ever saw your dog growling or barking at others when they are a puppy, often its due to their inexperience in reading another dog's language and them being not sure if the other is receptive to play. They will 'get' that. Some get it earlier than others but if you socialize - and do it often, they will get it earlier.
Just like us, they learn from their experiences - good and bad. We have to guide them down that right path of being totally amazing!