Reasons for dogs to suddenly become aggressive:

1. If the owner often hits the dog, the dog's temper will deteriorate and the dog will become aggressive.

2. Dogs are very territorial animals, so they will become aggressive when their territory is occupied.

3. It may be that the dog has encountered danger and cannot escape, and has been hurt by people, so it has become aggressive.

4. It may be that the owner carries the smell of other dogs, which makes the dog develop territorial awareness and suddenly become aggressive.

5. When approaching a dog that is eating, the dog may behave aggressively.

6. The dog is injured and in pain and cannot move. It may attack when people touch it.

7. Dogs may engage in aggressive behavior when trying to protect their owners, and the aggression will intensify when approached.

8. When a dog’s mood becomes bad, it will become irritable and aggressive.

9. They are hungry and may attack people in order to survive and get food.

10. Dogs will also become grumpy and suddenly exhibit aggressive behavior during the estrus period.

11. A mother who has just given birth to a puppy becomes very irritable and neurotic because she is afraid that others will hurt her puppy.