Harry loved when Sirius was calling him pup. He really loved. It was making him feel like he was wanted.

When Sirius was calling him like that he was so happy because there was someone who loved him.

Because it was making him feel as he had family.

As well when he was calling him pup he was petting his head. And that was amazing feeling.

Then he always had huge smile. He and Sirius. Both were like happy family.

But the biggest reason why he loved it, was because when Sirius wasn't calling him pup, he was saying James.