Final answer:
A sudden behavior change in an adult dog could indicate health issues, a neurological disease such as rabies, or results from lack of socialization. Symptoms like restlessness and aggression are warning signs, emphasizing the importance of social interaction and training for dogs.
Step-by-step explanation:
A sudden behavior change in an adult dog can indicate several possibilities including health issues, neurological diseases like rabies, or behavioral problems that may stem from improper socialization. Rabies is a particularly serious condition characterized by symptoms such as restlessness and aggression, as mentioned in Figure 26.1. Dogs that have not been properly socialized with others during their critical development period may exhibit fear or aggressive behaviors towards other dogs, as discussed in the LibreTexts extract. The author's tone of disapproval when describing the actions of an untrained, unrestrained dog, using words such as 'snarling' and 'stomped', highlights the importance of proper training and socialization in preventing such behavior.