The Second Fear Imprint Period occurs as dogs enter adolescence and seems more common in males. It is often referred to as adolescent shyness.
Your dog may suddenly become reluctant to approach something new or suddenly become afraid of something familiar. This behavior can be very frustrating to the owner and difficult to understand because its onset is so sudden and, seemingly, unprovoked. If you notice this behavior, it is important to avoid the two extremes in response: Don’t force him to do or approach something frightening to him or coddle or baby him.
To get through situations that make your dog fearful, be patient, kind, and understanding. Desensitize him to the object or situation by gradually introducing him to it and using food rewards and praise to entice him to confront the fearful object or situation. Do not coddle or reassure him in any way as that will encourage his fearful behavior. Do not correct him either. Simply make light of it and encourage him. Give him food rewards as he begins to deal with his fear better. Make sure you lavishly praise his attempts! This phase will pass!