Would you say that Danny's "obsession" would be helping/saving people? While we all agree Danny's the best boy, it does seem odd he immediately jumped to being a superhero at the expense of all other aspects of his life at 14. It also would explain why Vlad is still hung up on Maddie after 20 years & a billion dollars. The fact they're technically both living means their obsessions can grow and change over time, but they still have fundamental fixations at the expense of other things.
Actually, I'd say that's just a part of Danny's personality. Even without his ghost half, Danny made a conscious decision to get his powers back. He's scared of ghosts, but he wants to protect other people from them more than himself.
It’s the same thing for Vlad. It’s not like fixations are uncommon in DP. Jack and Maddie have ghosts, Paulina has the ghost boy, Valerie has her desire for revenge. Even Sam, Tucker, and Jazz have their fixations. Some characters even have more than one.
It could be something that comes with being around ghosts so much, or it could be the fact that they're cartoons. Cartoon characters can get a little extreme sometimes.
I tend to lean towards obsessions being the foundation of a ghost's psyche. For Vlad and Danny, that's their personalities. Being themselves is their obsession.
In a sense, you could say that makes them go a little farther with all their interests. Danny wants to help people, but because he's a ghost, he takes it too far. Vlad loves Maddie and resents Jack, but being a ghost makes him vengeful and obsessive. That's one way to explain things.
Or you can disagree with me altogether. This is only my perspective.