This theory states that the original Scooby-Doo cartoon took place during a period of economic depression. The economic depression was so terrible, that it even affected famous people like celebrities, professors, and athletes. Because of these set-backs, many of these normally respected individuals devoted themselves to a life of crime, and many disguised themselves as monsters so that they could shift the blame on someone else.
There is also another theory of Shaggy being a drug addict. He was a lonely boy who would go to school and was bullied by Velma, Freddy and Daphne all picked on Shaggy by calling him Shaggy and saying he smells and making fun of him in everyway. His family is quite rich and he is allowed lots of money so he buys drugs to get him out of his everyday life, in this drug reality Shaggy believes his dog, that recently passed away, is alive still and can talk, he also imagines that the three people that bully him are his friends, due to him using drugs he has the after effect of the 'munchies' that is why he eats so much. He also doesn't want other people to feel the same pain he has that is why he imagines stopping crime, or it is because he hides under a mask, because of this reality he is stuck in and these unmaskings are his brain trying to stop him from taking more drugs and get him out of this fake drug induced reality.
In his real life Velma would constantly call him a moron or idiot due to her high intellect and Fred would beat him up due to him being a jock and Daphne would say he smells and call him names as he was in her eyes 'ugly'.
The Gang is on the Run
There is a theory saying that since the first Scooby-Doo cartoon started in the late 60's it was set during the Vietnam war. And since the army was drafting teens at the time the gang was forced to runaway from home and relocate to Canada to avoid being drafted and going to war. Which explains why in every episode they're always driving in their van with no clear destination.
- Shaggy is your typical hippie with the case of the munchies, who is against war and decided to take his dog, Scooby on a road trip to protest this inhuman war.
- Fred Jones is a military school drop out who's draft number was up and he is AWOL.
- Daphne is (obviously) Fred's high school crush who can't bear the thought of Fred leaving and possibly getting killed in the war, so she joins the gang to be with Fred again.
- Velma is an anti-war activist but decides to join the gang on the road trip when a protest on a college campus turned into a violent riot.
Red Shirt Shaggy Theory
A question that is often brought up is "Why does Shaggy sometimes wear a red shirt?". The theory is that after events that haven't happened yet in the Scooby-Doo timeline, the group gets split up, and Shaggy, Scooby, and Scrappy travel the US doing temp jobs. Since the group split up, Shaggy had to take on a new identity, so he bought new clothing and no one noticed it was Shaggy all along.
Get ready because here comes some serious stuff. After those events to come, Shaggy blows all of his money on a new car to drive into the deep south to retrieve his inheritance in "Scooby-Doo and the Boo Brothers". After having taken all the money, Shaggy foolishly blows all the money on a different, newer car. He searches for a new job, finally finding one as a coach at Miss Grimwood's Finishing School in "Scooby-Doo and the Ghoul School". Shaggy is then spotted by one of his students' fathers', Count Dracula himself, and he turns Shaggy into a werewolf so he can compete in his savage, barbaric, deadly race. Shaggy escapes and returns to human form, but enters a time vortex in the process. He is then sent back to the past, the future Scooby-Doo movies, which is why he's in the same economic state in Zombie Island.
In conclusion, Shaggy is a slave to time travel, and must live out an endless existence in full loop, never tasting the sweet taste of death. Every time you watch a Scooby-Doo movie, it's just another iteration of his endless existence, all for your petty entertainment.