If you notice a sudden increase in fearful behavior from your dog, it could be because they have entered the secondary sensitive/fear impact period. This is a normal phase when dogs may become suddenly fearful of specific dogs, people, or objects, typically occurring from 6-16 months old. Fearful behavior can also occur due to environmental and genetic reasons such as under-socialization or a bad experience in a particular situation. If your dog continues to be fearful for over two weeks, it’s best to contact an animal behaviorist.