This theory states that the original Scooby-Doo cartoon took place during a period of economic depression, making sense of why every place they visit is run-down. This economic depression was so terrible that it even affected famous people like celebrities, professors, and athletes, as well as experts in fields like robotics and special effects. Because of these set-backs, many of these normally respected individuals and individuals with highly marketable job skills devoted themselves to a life of crime, often disguising themselves as monsters to evade blame. This also explains why the teenage characters are never seen in school; all schools have shut down. Additionally, since the gang is always in their van with no clear destination, it is possible they are homeless and living in the van.
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, who all teased by calling him Shaggy, saying he smells, and generally making fun of him in every way. 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 his drug-induced delusion, Shaggy believes his recently deceased dog is still alive and can talk. He also imagines that the three people who bully him are his friends. Due to him using drugs, he has the after-effect of the 'munchies', explaining why he eats so much. He also doesn't want other people to feel the same pain he has, and that is why he imagines stopping crime. Alternatively, perhaps he hides under a mask because of the 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, Fred would beat him up due to his higher status as a jock, and Daphne would say he smells and call him names because she found him '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. Because the army was drafting teens at the time, the gang was forced to run away 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 a case of the munchies. Being against war, he decided to take his dog, Scooby, on a road trip in protest.
- Fred Jones is a military school drop out whose draft number was up, making him 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.
- Velma is an anti-war activist who decides to join the gang on the road trip.
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. Simply buying new clothing was enough for him to go unrecognized.
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 and 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, who turns Shaggy into a werewolf so he can compete in his savage, barbaric, deadly race. Shaggy escapes and returns to human form, but he 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.
Each Member Represents One of The "Five Colleges"
The five main characters each have the characteristics associated with one of five eastern colleges: Amherst, Hampshire, Mount Holyoke, Smith, and University of Massachusetts Amherst (UMass) . Fred is the sporty and preppy Amherst, while Shaggy is the aimless slacker druggie stereotypical of Hampshire . Meanwhile, Daphne is put-together, stylish, (and, theorists presume, a husband hunter) and pretty like those thought to typically attend the upper-class Mount Holyoke, and Velma is more like Smith with her serious, slightly frumpy, and studious librarian look that doesn't exactly suggest she's overtly concerned about courting male attentions . Finally, Scooby immature, hyperactive, and silly party animal like those at UMASS .
- Creator supposedly went to one of those schools 
- The creators probably modeled them after characters from The Many Loves of Dobie Gillis  There were other concerns beyond conforming them to stereotypes in play when shaping the characters .