1, territorial dispute
Dogs are pets that occupy their territory. Even dogs that grew up together will divide their own territory.
If this occupied place is "missed" by another dog, there may be a fight to regain its own territory.
2. Confirmation of grade status
Dogs have a certain sense of hierarchy, which is inherent in them. If a dog was originally kept at home, and then another dog joined in, they would fight to confirm their status.
The winner plays the leading role and the other side has to obey.
Step 3 be in estrus
Dogs in estrus will change their personalities and may become grumpy, especially male dogs.
If dogs in estrus fight with many male dogs, they are all fighting for mating rights.
Step 4 be alert
Most dogs are possessive of their owners. If they find their owners touching other dogs, or other dogs approaching their owners, dogs will feel jealous, and some dogs will attack another dog because of jealousy.