Whereas eight-week-old puppies are universally accepting of people, adolescent dogs naturally become wary of anything unfamiliar, including noises, objects, dogs, people and places. It is not uncommon for adolescent dogs to become fearful or reactive. As puppies grow older, the world becomes a scarier place.
To prevent dogs from becoming wary of children, men, strangers, skateboarders, other dogs, loud noises, vacuum cleaners, nail clippers, collar grabs, etc. etc. etc., take your time when exposing your puppy, adolescent, or newly adopted adult dog to novel (unfamiliar) stimuli, settings and situations. And make sure you condition your dog not only to tolerate, but also to thoroughly enjoy all of these potentially scary stimuli.
*It is easier to condition BEFORE your pup shows any fear, both as he grows older, and during the training.