The Critical Period

Between about three and fourteen weeks old, a puppy's brain runs in a special mode. New things get filed as "normal" almost automatically, with little fear attached.

Once that window closes, the brain flips its default. Anything new now reads as a possible threat instead of an interesting puzzle.

This is why gaps show up later as oddly specific fears. A dog who never met a man with a beard as a young puppy may bark at bearded men for years. The brain simply never got to file that face under "safe."