Aggression between dogs during socialization is a common problem, and it can be caused by various factors. Some dogs are naturally more dominant and aggressive than others, while others may have learned aggressive behavior from negative experiences or a lack of proper socialization during their critical period of development. Here are some of the most common reasons why dogs show aggression towards other dogs during socialization:

1. Fear: Fear is one of the most common reasons why dogs show aggression towards other dogs. If a dog feels threatened or intimidated by another dog, they may react defensively by growling, barking, or even biting. This can be especially true for dogs that have had negative experiences with other dogs in the past.
2. Lack of socialization: Dogs that were not properly socialized during their critical period of development may not know how to interact appropriately with other dogs. They may be overly fearful, defensive, or aggressive towards other dogs, as they do not know how to communicate effectively.
3. Dominance: Some dogs may be naturally more dominant and assertive than others. These dogs may try to establish themselves as the alpha dog during socialization, which can lead to aggressive behavior towards other dogs.
4. Resource guarding: Dogs may become aggressive when they feel that their resources, such as food or toys, are being threatened or taken away by another dog.
5. Medical issues: Some dogs may show aggression towards other dogs due to underlying medical issues, such as pain or illness.

It’s important to note that aggressive behavior during socialization can escalate quickly and can be dangerous for both dogs involved. It’s essential to intervene and remove the dogs from the situation if there are signs of aggression. It’s also important to work with a professional dog trainer or behaviorist to address the underlying cause of the aggression and to develop a plan to help the dog learn appropriate socialization skills.

In summary, aggression between dogs during socialization can be caused by various factors, including fear, lack of socialization, dominance, resource guarding, and medical issues. It’s essential to intervene and address the underlying cause of the aggression to prevent further escalation and to work with a professional to develop a plan to help the dog learn appropriate socialization skills.