Why do schizophrenics hallucinate bad/scary things? Why can't they hallucinate nice/fun things?

I don’t get it. I mean, there’s got to be some explanation as to why they hallucinate things that makes them self-destructive. Do you know or have you heard of any one hallucinating things and actually enjoying their hallucinations? Because most of those I know, either want to kill themselves or others. Why do they hallucinate the bad stuffs and no fun stuffs?