Minerva rarely went to the Gryffindor common room unless there was something very important. She considered it a matter of respecting the students' privacy.
Pomona, on the other hand, often took the opportunity to check in by replacing the singing magical plants in the common room. Among boarding students without parental supervision, left to act entirely according to their own preferences, she believed that appropriate intervention could prevent some serious incidents. Besides, children going through adolescence often had all sorts of worries and troubles, and she was willing to listen. Flitwick, by contrast, basically left everything alone. As he had once said, he kept his distance from humans.
The rule requiring children to buy pets had left Pomona with a bad impression. Though the rule had originally been established because some Transfiguration lessons required turning animals into things like glass goblets.
Once there were pets, comparisons were inevitable. The rarer the pet, the more attention and gawking it attracted. Who would pay attention to a turtle, or an ugly lame cat?
Ron suspected that Hermione's pet, Crookshanks, had eaten his pet rat, Scabbers. More than once, he openly accused her in public and demanded an apology.
But Hermione firmly refused to apologize. She believed Crookshanks had merely done what a cat was supposed to do.
It was not enough to damage their friendship, but it did create conflict and disputes. Thinking of pets reminded Pomona of Prickle, the pet she had given to "Severus." It was a magical creature highly wary of humans. Severus had put a great deal of effort into making it friendly toward people, but that change brought it no benefit.
Severus's "mistake" had been getting close to a Gryffindor girl, and then he had been "punished."
After that, Severus never kept another pet. At least Ron