The intense popularity of the original Yu-Gi-Oh! series prompted multiple spinoffs with new casts of colorful characters, including a sequel series, Yu-Gi-Oh! GX. The burning question behind the GX series was "Who will be the next King of Games?" — a question that protagonist Jaden Yuki wished to be the answer to.
This should be an simple enough wish, thanks to the activities of the original cast, there are no evil entities after the Millennium Items, the Items themselves are absent, the Egyptian God Cards are de-powered and all the persistent villains are defeated. Jaden and company should be happily living in a post-Item world where ancient entities are no longer a concern, and earning the title of King of Games should be no harder than mastering the (admittedly difficult) game of Duel Monsters. Right?
Well…no. Because as it turns out, the cast of GX had their own laundry list of problems involving ancient entities and crazy villains. From the overarching villain Darkshroud to the malevolent Light of Destruction, this fresh group of teens had plenty gunning for them — but none so personal as Yubel, the obsessive, possessive Duel Spirit who wanted Jaden all to themselves, seeing as how a previous incarnation of Jaden promised them so.
Yubel had but one desire: to have Jaden to themselves and keep him safe always. Their interpretation of this was to get rid of all of Jaden's friends and cause him as much pain as possible, the latter of which was because Jaden had them sent into space. Yubel's actions — if the supernatural parts are removed — smack hard of an abusive relationship, from the insistence on isolation to blaming Jaden for everything bad they're doing. It's the sort of thing that makes their eventual redemption questionable, especially considering it came at the cost of Jaden fusing with them permanently via the card "Super Polymerization"— thus granting their wish.
Granted, in doing so Jaden also purged them of the villainous Light of Destruction, but it's worth noting that Yubel was putting his friends into comas before coming into contact with the Light, because if he's playing with his friends he's not spending time with them. Also worth noting is that as a Duel Spirit, Yubel can only be seen by certain people — which means that Jaden is isolated by an entity he can't explain to anyone who doesn't also have the very rare ability to see Duel Spirits. This is the case straight up to the events of Season 3, when Yubel transports the entire Duel Academy to the Duel Monster's world and reveals themselves to the entire school — which begs the question: why would anyone want to play Duel Monsters after this?
It was established in the first series that Duel Monsters are indeed alive and cognizant, inhabiting their own world and crossing over into ours when summoned through a duel. For those involved in the Shadow Games, these monsters were painfully real — but in the first series, this affects a very small group of people. The Waking the Dragons Arc brings this awareness to a global scale, but the impression from the average person is that KaibaCorp's duel disks are malfunctioning, not that their monsters are now alive. But in GX, not only do they have the whole Academy being transported via a Duel Monster that the whole school sees, but the majority of the student body is put in grave peril thanks to this monster. One would think the revelation that the cards they so casually play with house real monsters that can do real damage would cause a mass scramble for a new job field.
Especially when it becomes clear that these monsters can do emotional damage as well as physical. Jaden only ends Yubel's reign of terror when he professes his eternal love for them — everything he and his friends tried before to dissuade them or stop them only ended with them coming back stronger. It's questionable whether Duel Monsters truly die, and Yubel not only survived everything the students threw at them, but also re-entering Earth's atmosphere. Yubel is an implacable monster who refused to stop until they got what they wanted.
And what's to say that Yubel is the only possessive spirit out there? What about those who can't see Duel Spirits? How does a kid feel, wanting to play a supposedly innocent card game and yet having horrible things happen every time? Nothing they say can explain it, only that it happens when they play Duel Monsters. The whole game suddenly takes a hard turn into horror, turning it into something dark and cursed — ironically, precisely what it was to begin with.
Even discarding the Duel Spirits, a lot of honestly horrible things happen around Duel Monsters. From the ancient Shadow Games to modern-day villains using the game to their own ends, asking why anyone is still playing this game is a seriously valid question (and makes Paradox's desire to rid the world of Duel Monsters just a tad understandable). Granted, the events of the last few arcs of the original series may have been what prompted Seto Kaiba to create the Duel Academy: he wished to arm the next generation against such evil, sort of a self-defense degree, if you will. But at the same time, it's a lot to ask of a bunch of kids just trying to have some fun with a card game. Between the propensity of villains to use this game and the capacity of the cards themselves to wreak havoc, it kind of seems like just not playing the game is the only way to win at this point.