When Danny adopted Cujo he saw him as a puppy, which was a little sad considering the implications. He always thought Cujo was the only one who had stayed, the only one whose soul had persisted. He thought Cujo was alone.
He was wrong. Not all adult dogs and puppies stayed, of course. But many had. Cujo was simply the only one who decided to return to the world of the living, probably because of his obsession with his toy.
One day Cujo showed him where they were all hiding; Danny knew that those dogs had not stayed for the same reason as Cujo, they probably had a myriad of different reasons, and that was fine.
The problem was that excluding some of the adult dogs (that obviously were fine on their own and didn't care), there were many puppies similar to Cujo running around in need of affection and he couldn't adopt them all (besides, Cujo would definitely get jealous). And while many ghosts agreed to take a couple, it wasn't all of them, so Danny did something extreme.
He held an adoption fair in Amity, which was a smashing success. He just forgot that a lot of people in Amity...were usually traveling, and the ghost puppies would follow.
Then, a few days later when a scowling guy showed up (he obviously wasn't part of the general Amity Park population) and demanded a "bright green" puppy, Danny said no and refused his application. He couldn't trust someone with no knowledge with a ghost dog. Although he did offer him a course to learn about their care.
Damian Wayne was offended with his overall assessment. He was obviously the right person to care for one of those pups. So he set out to prove that to the boy in front of him, without hesitation.