Even when you aren’t actively training your dog, he’s always learning. Think about all the things he’s figured out on his own, like when you’re about to go out or where the dog treats are kept. That means every interaction teaches him something new regardless of your intentions. And the same is true for his interactions with others.
So, if you’ve decided that your dog is not allowed on the furniture, but other people cuddle him on the couch, what can you expect your dog to learn? How can he understand which of those two things is the rule and which is the exception? It’s understandable that your dog will be confused, and your training will stall. Or if you want your dog to only walk on your left side, but your dog walker lets him wander back and forth, how can he understand which behavior is correct. Both situations are learning opportunities, but more importantly, the one that most gets him what he wants will be the stronger lesson. The cuddle on the couch will have more impact than having to lie on the ground.