Shaggy being a marijuana smoker, based on his being portrayed as constantly hungry, usually shown wearing a dirty t-shirt and a scraggly beard, riding around in a psychedelically painted van, and having conversations with his dog. The Movie plays with this. An episode of Harvey Birdman, Attorney at Law also played with it, lampshading the theory when they get falsely arrested for being pot-smokers. Jay and Silent Bob Strike Back also plays with this: Jay and Silent Bob are picked up by the Scooby-Doo gang and turn them on to "Doobie Snacks."
For that matter, is Shaggy really a "lazy beatnik" or is he the Only Sane Man who wants to do the sensible thing and let the law enforcement handle the problem? Adding on to this, we only really see him when he's tired and annoyed about being somewhere he's not supposed to be doing something he doesn't really want to do in the middle of the night. In the 80's series he steps up to the plate with relative ease. In fact, he might just be one of the smarter members of the gang and among the most capable: In Scooby-Doo and the Ghoul School he's a teacher which requires a college degree to be certified as, and fairly good at said job for that matter, and has a number of additional talents including being a champion mini-golfer. Many of which, as should be noted, are daytime activities when he would have more sleep than the typical scooby chase scene.
Daphne could well be Silk Hiding Steel. IRL, getting kidnapped in the middle of the night once by a guy in a creepy costume would be quite traumatizing, but Daphne shrugs it off and continues trying to help. She also does quite well leading Mystery Inc. in Fred and Velma's absence.
The most popular interpretation for the other characters is that Fred and Daphne always run off together to have sex, and Velma is a lesbian. (Velma being a lesbian started in reference to Sheila James Kuehl, the actress who played Zelda Gilroy, a character from "Dobie Gillis" on whom Velma was based. Keuhl was the first openly gay person to be elected to the California Senate.)
Alternatively, Velma is straight. But she's traveling with a dog, a pothead, a lesbian, and a gay man (the last two acting as beards for each other).
Why does the gang stick together, and why is it named the Scooby Doo Detective Agency? Fred, for whatever reason (paranoia, misguided sense of justice, just wanting to impress Daphne), wants to solve crimes. Of course he has to take Daphne, she's the one he's trying to impress. He knows that Velma is both smart and into him, so he gets her to come along to give the operation some legitimacy. But that still doesn't explain Shaggy and Scooby - they rarely make an effort to contribute to the investigations, and often are a detriment to the team, especially considering their low morale. The answer is simple: Shaggy is a pot dealer, and he owns the van. That's why they never tell him to take his dog and leave: because without him, the whole operation falls apart.
On top of that, some speculate that Scooby can't really talk, and his vocalizations are all in Shaggy's head from eating too many Scooby Snax...which are really "special" brownies.
Heavily and played with thematically in the The Venture Bros. episode "¡Viva los Muertos!" Not only are the "Groovy Gang" re-characterized as several kinds of crazy, they're also tied to actual crazies, with Daphne renamed Patty, and drawn scripted to resemble Patty Hearst, and Shaggy/Sonny modeled on David Berkowitz, the serial killer known as the Son of Sam, and Scooby/Groovy on Berkowitz's neighbor's dog Harvey that Berkowitz claimed was possessed by an ancient demon and commanded him to commit murders.
Scrappy's ego size. Several detractors (and a few fans assume that from the way he constantly throws hiself int a fight points to an oversized one. Though others argue that he he does it because he's insecure about his placement with the others (supported by a rather...unique Scrappy and Yabba short) and that he feels he needs to constantly do difficult or dangerous jobs so his caretakers don't abandon him. His constant forthright tactics are a result of an overachiever nature and low self-esteem.
Is he oblivious to Scooby's cowardice? Or is he in denial? Or is he trying to bolster his confidence?
The 2002 Movie
Scrappy-Doo could be a sympathetic villain. In the cartoons, he idolized his Uncle Scooby; in the movie, he wanted to destroy humanity with a plague of monsters. Maybe he found out what a coward Scooby was and that his own parents didn't want him around. He may have a genetic disorder that kept him at midget canine status, which made him twisted when he grew up. The bloated ego in the movie could be his need for attention and affection.
A lot of people just assume that Velma's flashback was accurate since Scrappy DOES show up later as the villain and he and mystery inc. are as unfriendly with each other as her flashback portrayed. However, even with that in mind, a lot of details of the flashback is still pretty questionable, even if you don't take any other piece of Scooby canon into account:
For one, the gland disorder detail doesn't make sense when you know that gland disorders that stunt canine growth actually do a lot more than just make you smaller: To put it simply, Scrappy'd be in no shape to take over the world, much less be strong enough to get up on his hind legs to walk. And if he had any gland disorder, his behavior matches up best with one that wouldn't affect his size, because his mannerisms match excess of growth hormones.
Scrappy's infuriated yells are audible to the viewer as the flashback ends, but Velma, the one who is delivering the flashback, wouldn't have been able to hear clearly since she was in the van with the doors shut which was moving away.
The real Mondavarious, who also is probably an unreliable narrator Given he's been just been freed from a dark hole he's been trapped in for an indefinite amount of time, but he states that it was approximately two years ago that Scrappy showed up, which means Velma's rose-tinted flashback would have happened not long before the opening (and mystery inc's) break-up happened.
Also, keep in mind that, if Velma is telling the truth about abandoning him in the middle of nowhere, it's in her best interest to make her side come off as the good guy. And with nobody else in Mystery Inc. present to have to corroborate with, Scrappy rotting in Nevada somewhere (so she thinks) she has as much free range to put her own spin on how things went down. (As well as Spooky Island's trademark 'bloody skulls' working their magic.)
Even more interestingly, The novelization, where Velma says that she only had a mixture of tomato and celery, her story is different.: All of Scrappy's misdeeds except for appointing himself their unquestioned leader, for one, are omitted. Velma only states that they all found him to be a pest until one-day Scrappy randomly (with none of the Urine Trouble or "scrote" talk mentioned) decided to appoint himself their unquestioned leader. She only states that they hit their limit and "just left him standing there", which could easily have been in a much safer, less cruel place to ditch him then in the middle of Nevada. Even more interestingly, she doesn't mention Scrappy bringing up leaving of his own free will if they refuse...
Is Mystery Inc. getting back together supposed to be a happy ending? While they all reconcile with each other, none of the issues with the gang (Except for Fred and Velma's dispute) are actually addressed in depthnote And it's up in the air whether or not Fred can continue to work at being a more considerate person and not fall back into his old glory hogging habits
Shaggy and Scooby still hate getting into danger and yet are still thoughtlessly dragged into the mix.
Daphne now can defend herself, but aside from Fred getting angry at Scrappy when he peed on her her friends are all pretty frosty towards her many scrapes with those that she would need to defend herself from, nobody bats an eyelash when she complains that Old Man Smithers sexually harassed her, after not seeing her for two years they react dismissively and condescendingly to her claim of training in martial arts, (Especially egregiously, nobody questions Velma going from amateur vigilantism to a scientist at NASA nor Fred, a narcissistic Cloud Cuckoo Lander getting a publishing deal) and all Velma has to say in response to Fred gleefully and remorselessly talking about looking at her body naked while he and Daphne have swapped bodies is a mildly irritated, "oh, brother"...really, aside from the aforementioned Urine Trouble debacle with Scrappy they are pretty apathetic towards her rather troubling plights, which is left unresolved. Worse, nobody actually sees her climactic fight with Zarkos.
Mystery Inc. seems to have a Toxic Friend Influence upon each other: when they're apart, Shaggy and Scooby, while they are pretty broken up over it at first, are eventually seen having a blast beach bummingnote Maybe not the "average" job, but they have no problem obtaining plenty of food and they both seem to be living idyllic, content, and care-free lives before being heavily bribed back into the adventure, Velma managed to get a job at NASA in two yearsnote An earlier version of the movie had her actually been in therapy, but even so, it's still a form of self-care that she doesn't seem to do when she's with the gang, only after breaking it off from the others does Daphne finally take the initiative to learn self-defense after what is implied to be multiple kidnapping incidents and snide remarks from her friends, and Fred definitely isn't complaining about having all the time in the world to write self-aggrandizing autobiographies. When they are together, Shaggy and Scooby are constantly terrified and in lethal danger, Daphne is attacked/poorly dressed for the occasion(and in lethal danger), treated with indifference, Fred is off in his own head (and in lethal danger) and Velma seems chronically frustrated with everyone (and...well, guess). And they spend all their time fumbling their way through a dangerous heist their ill-prepared and unobligated to get mixed into and don't get shown getting compensated for.