Reason behind Dr. Herriot's Severe Tone towards Mrs. Pumphrey
Dr. Herriot, a veterinarian, adopted a stern tone when giving strict instructions to Mrs. Pumphrey regarding her pampered dog, Tricki, for several reasons:

Professionalism
As a professional in his field, Dr. Herriot understands the importance of clear communication and setting boundaries with pet owners. By sounding severe, he aimed to convey the seriousness of the situation and ensure that Mrs. Pumphrey would follow his instructions carefully.

Concern for Tricki's Health
Tricki's obesity and resulting health issues were a cause for concern for Dr. Herriot. By using a strict tone, he wanted to emphasize the critical need for Mrs. Pumphrey to adhere to his advice in order to improve Tricki's health and well-being.

Establishing Authority
In dealing with overly indulgent pet owners like Mrs. Pumphrey, it was important for Dr. Herriot to establish his authority and expertise. By appearing stern and serious, he aimed to assert his position as the professional in charge of Tricki's care.

Ensuring Compliance
Dr. Herriot knew that in order to make a real difference in Tricki's health, Mrs. Pumphrey needed to fully commit to following his instructions. By using a severe tone, he hoped to impress upon her the importance of compliance and the serious consequences of not taking his advice seriously.
In conclusion, Dr. Herriot's decision to sound severe when giving instructions to Mrs. Pumphrey about Tricki was driven by his professionalism, concern for the dog's health, need to establish authority, and the importance of ensuring compliance for the well-being of the pet.