If I can believe in spirits, why can't I believe in other mythical creatures?

I've always questioned myself this. I believe in spirits. My friends believe in them. Then I think about it..why can't I believe in, I don't know, fairies? Mermaids? Dragons even. All these creatures are mythical, but why is it that I can only believe in spirits?

Spirits tend to be associated with all things negative. Then again, so are mermaids, with their ability to cause storms and shipwrecks. Some people believe fairies were once pure evil. Dragons can kill, I mean they're meant to breathe fire. They're all deadly - minus the fairy - and horrifying

I mean, I grew up thinking spirits were just a myth, same with fairies, unicorns and so on, but as I grew older, I chose to believe in spirits. A part of me does wish that I could believe in other mythical creatures..but why can't I..?