Because she give everything to Tricki to eat. He gain fat day by day. It was not good for his health. So it was important for Mrs. Pumphrey to be little cruel towards Tricki.

Thanks to both of you
Excess of everything is bad. Mrs. Pumphrey pampered trickie too much. She was an exceedingly pampering woman; her pampering spoilt her pet dog, Tricki. She had provided all types of facilities to the dog such as day bed, night bed, favourite cushions, toys and rubber rings, breakfast bowl, lunch bowl, supper bowl, etc. In her love for him, she let him overeat; she did not make him do exercise. As a result Tricki became exceedingly fat. His health suffered. He had to be admitted to James Herriot’s clinic where he treated him by not giving him much to eat and making him do lots of physical exercise.
Excess of everything is bad. Mrs. Pumphrey pampered trickie too much. She was an exceedingly pampering woman; her pampering spoilt her pet dog, Tricki. She had provided all types of facilities to the dog such as day bed, night bed, favourite cushions, toys and rubber rings, breakfast bowl, lunch bowl, supper bowl, etc. In her love for him, she let him overeat; she did not make him do exercise. As a result Tricki became exceedingly fat. His health suffered. He had to be admitted to James Herriot’s clinic where he treated him by not giving him much to eat and making him do lots of physical exercise.