Sorry, I don't like criticising trainers, having myself been a trainer instructing both at dog training clubs and breed clubs for over 50 years, but that trainer is talking rubbish!!
I think it’s important that you understand whats happening. What you describe is so normal for Labrador puppies! Part of the problem is that they are so much a social breed, they love everybody and want to be with them. They want to play, and they want to involve their “human” in their game. And of course dogs cant play cards or computer games. Puppies are pre-programmed at birth to play “War Games.” This is equipping them for their future in the wild, catching their food and defending themselves and their pack. Puppies, and many other creatures in the wild will practise and hone their skills on each other. I’m so lucky in that I have a private wood that I can walk in and often in spring I’ve stood and watched Fox cubs playing these war games. They have no intention of hurting each other, just have a lovely game.
But then onto domestic dogs, and thats where things all go wrong. We take our puppy out of the nest and away from his siblings at around 8 weeks old, just about the time when the pups are beginning to get active, starting to think about things other than eating and sleeping. So his natural actions now would be to play his war games, but he has no siblings to play those games with! So effectively you are the surrogate sibling! In the wild this is where he would start to learn bite inhibition. As a baby he would have no idea that biting hurts! How could he? So he nips his sibling a bit hard, brother says, “Oye! Pack it in! If you are going to play rough I’m off!” In other words he walks off and leaves his brother. Brother soon works out that biting too hard hurts and finishes the game. Particularly if it’s him that gets bitten too hard! So the pups start to learn to control their biting.
Why do they single out one particular person? Because they think that person is nice, so they want to play, and play in the only way they know. So really, much as you done want it, it's really a compliment! Your puppy feels happy and confident with you.
This is where the theory of “Time out” came from. It’s us trying to replicate what would happen with puppies naturally in the wild. “That hurt! I’m not playing any more!” So you stand up, turn your back on the pup, get your hands up high so there is nothing for the pup to take hold of. But you have probably noticed that things are worse in the evening. Just like children, they can lose a certain amount of self control when they get tired. All day you are busy so they spend a large part of the time sleeping, but in the evening, when you want to sit quiet, resting from the day they want to play. And as they get tired so the play gets rougher. I have always made a point of popping my pups into their crate at about 7pm for an hour, so they get use to having an hours sleep in the evening, and I get a chance to recharge my batteries. Interestingly this has built a habit which has continued for all of my dogs lives. Every evening they put themselves to bed and we all have a rest.
I know puppies are hard work, and the alligators can make your hands really sore, but believe me, it does get better. Yes my dogs still love to involve me in their games, but it’s now lovely. Amy takes hold of my wrist so gently and leads me to where she wants to go, or Chloe will take hold of one finger to involve me in her war games, but oh so gently. Somehow you never notice things getting better because it is a slow change in pressure. But one day you realise your hands are no longer sore and you cant remember the last time you told him to pack it in. Given time they become the most wonderful of creatures.