1. Puppies are hardwired to stick close for safety. It's easy to be lulled into thinking that you have either finished training, or that your dog is perfect and naturally stays close-- don't be fooled!
2. Adolescents are hardwired to explore the world because in nature they would be preparing to head out into the world on their own. (No one told your dog's genes that you plan to keep and care for them forever).
3. Your puppy did not notice a lot of the things in the external world. Suddenly, almost overnight, the puppy notices the environment