Puppies rapidly learn to be aggressive if it is in their best interests to do so. Situations where the puppy is placed in a situation where it feels threatened can rapidly lead to warnings and, eventually, snaps and real bites if these are ignored. Fear-based aggression is the most common reason why puppies under 6 months of age are aggressive to other dogs and puppies. Puppies do not have to have bad experiences with others to be worried about them. No experiences with other dogs early in life will cause them to worry later when they see something they are not familiar with. Consequently, if your puppy has not had adequate encounters with other dogs and puppies, he is likely to see them as a threat and react defensively to them, particularly if they approach fast. Some puppies are worried about other dogs because they have had unpleasant encounters with them. These experiences do not need to be painful for a puppy to try to avoid them in future, getting a fright can be just as bad.