Sirilla could not imagine what it had been like for Sirius when he was unable to transform. There had been a time in her life when she hadn't been able to do it, of course, but that had been when she didn't know what she was missing. Once she had discovered her inner canine, she could not imagine life without it. She couldn't imagine life transforming into anything else, either. What did people do when they didn't have a wet nose, a lolling tongue, and a wagging tail?
She was intensely happy for him having Padfoot back, and completely understood the desire to spend as much time in dog form as possible. And the dog was never happier than when it was running and playing with its friends-- they were both used to wolves and deer, but there was a beautiful play of communication that was instinctive between two dogs. Rough play with soft mouths, exploring their surroundings with their noses, reveling in the simple joy of the run.
Even once she transformed back, she could still feel the dog's adrenaline and bubbly happiness running through her veins, and she rested happily against Sirius's side. After a moment she shifted to rest more comfortably against him, because he was softer than the tree, resting her head against his chest.
"Me too," she said. "I love Hogwarts. I'm gonna miss it." She wrinkled her nose. "Won't miss the Slytherin dorms, though."