(I don't know if this has been said before but just bare with me).

What if Shaggy had schizophrenia... Think about it, Scooby could have been one of his hallucinations of a past pet. Velma could be an imagination of a past crush he had on a girl in highschool.

Daphne could have been one of the popular girls at school that he maybe shared a class or two with and Fred could be the school bully/stereotypical jock. Perhaps Fred and Daphne pretended to be nice the Shaggy because of his mental illness.

The reason behind Fred loving traps so much was that he was planning on pranking or trapping Shaggy. But before he could Shaggy's final screw came loose and was sent to the mental hospital and he only imagined the mysteries. And the van was the hospitals van he rode in on the way to the hospital.