Hatori gets this too mainly due to his Base Breaker status. Does he truly love Chiaki for who he is or is he want Chiaki just to rub into Yanase's face. Chiaki certainly thought this in episode 5. Also, given the other episodes including episode 16, Hatori does come off as an extremelypossessiveboyfriend especially episode 10.
Base Breaker: Hatori gets a lot of flak. He's either the perfect candidate for Chiaki because of how he takes care of him and known him for twenty years or is hated because he's a possessive seme that will use force to hurtChiaki and Yanase. Episode 16 doesn't help the cause. Same thing applies to Chiaki though. He's either the cutest uke ever or the dumbestuke that needs to see that's he's hurting both of his friends because of his idiotic nature.
Die for Our Ship: You can guarantee that if you're a Yuu/Chiaki fan, Hatori will almost always get bashed and written as pure evil, kill him off so Yuu can have Chiaki for himself or be written out completely to give Yuu a chance at love.
Draco in Leather Pants: While definitely not a villain, the fans that defend Hatori for his actions (in the novel concerning him raping Chiaki) being justified in the since that he needed to do this in order to get Chiaki to realize that he had feelings for him. You can see how everything is divided.
Ensemble Darkhorse: Yuu Yanase is generally more liked than Hatori or Chiaki in terms of the love triangle. Despite being the character on the short end of the stick, he stands out a lot due to how his personality is different from what Nakamura usually writes. His voice actor and personality is very similar to a certain troll from another anime helps his case.
Yokozawa went from being one the least liked character when first introduced to one of the most beloved characters after getting his own case even more than Onodera. (He even got his own movie).
The first Yokozawa novel was published in Nov 2011 and there's already going to be a sixth novel released on Oct 2014.
It's better to understand why after reading about him getting over his heartbreak with Takano and his new budding relationship with the Kirishimas, Zen Kirishima especially.
Epileptic Trees: Many fans who have read Hybrid Child, another of Nakamura's works adapted into an anime, widely speculated that Takano and Ritsu are the reincarnations of Kuroda and Tsukishima, who are one of the three pairings featured in the aforementioned manga, due to their physical resemblances and their similar relationship dynamic.
Fan-Preferred Couple: A more minor example in that some people would prefer Chiaki with Yanase than Hatori. This has to do with Hatori brutally raping Chiaki and episode 16 that pulled everyone's heartstrings.
Idiot Plot: Episode 16 is regarded as one given that it focuses on Domestica. So Hatori cancels a date with Chiaki to meet up with his ex-girlfriend about rooms for two people. Chiaki ends up assuming that Hatori is cheating on him again (as he already assumed this in episode 6 when they first started dating), and what does he do? He goes complain to Yanase about Hatori "cheating" on him when in the last episode, Yanase has made it clear to Chiaki that he's in love with him and wants to be more than friends. While things don't end well for Yanase (and he's not really at fault in this episode), Yanase's breakdown could have been avoided if...
a. Chiaki asked Hatori why he cancelled the date instead of assuming he's cheating on him. (To be fair, Hatori's ex was being too familiar with him and Hatori didn't explain her actions nor did Chiaki bring it up).
b. Hatori just told Chiaki what he was planning instead of keeping it a secret.
Internet Backdraft: Do not mention the Hatori/Chiaki pairing online and your opinion about the gruesome rape in the novel if you wish to save your heart from being broken.
Jerkass Woobie: Takano because he had a crappy family life and when he found out that Ritsu (the only person that cared for him during his high school years) had a fiance and left him shortly afterwards, he had a breakdown and Yokozawa had to pull him out of it. Naturally, he ends up being a huge jerk when he finally recovers but once the audience (and Ritsu) finds out about his past, you will feel sorry for him.
Yokozawa too would be in this category especially after Takano rejects him one last time telling him not to interfere with his life. Takano is talking to the person who pulled him out of his despair and had been waiting for years to end up with him only to be shot down cruelly.
Ritsu too would be in this category as the misfortunes he has is caused by him being in denial. Also Takano's breakdown in high school is hinted to be Ritsu's fault and not the other way around, though he MIGHT be forgiven, if one would assume that Takanos "The Reason You Suck"-speech might have left him even more insecure then ever. However, we know that Ritsu also has moments like being looked down by his workers at his father's company because he's the son of the cooperation, and being talked behind his back as a result. He also doesn't get any respect from his worker and Yokozawa constantly harasses him to stay away from Takano when all he wants to do is get his life back together. Not to forget how his mother constantly harrasses him by how he isn't all over "An-chan" which only gets worse when An-chan actually breaks the engagement, ending with his mother pretty much announcing that he is fully to blame, that she will find a new fiance despite his wishes and also hinting on Ritsus father not even showing any interest in his life, sending him into a minor BSOD.
And of course Kisa. It's easy to feel sorry for him however, he causes most of the problems in the relationship with Yukina because of his pessimistic personality. However given his age and how he's given up on life (in terms of finding happiness because he feels like he's behind in everything), you can't help but pity him. Luckily, someone like Yukina can pull him out of his depression.
Mind Game Ship: Takano and Onodera in high school. In the last episode of the anime, Takano notes that he started to go out with Onodera...to crush his illusions.
Moral Event Horizon: To a lot of people, Hatori crosses the line when he brutally raped Chiaki in the novel. The anime was nice enough to tone it down but said fans aren't going to let what happened in the novel slide and it implies it in the anime as well. Being a Karma Houdini and beating up on Yanase does not help his case in trying to redeem himself.
Never Live it Down: Hatori gets a lot of flak from people because he raped his childhood friend after Chiaki begged him to stop and he didn't even listen. Mind you that this is one of the few times where Rape as Drama instead of the typical Rape is Love. The fact that the scene is never mentioned again pisses a lot of people off.
Chiaki may not be the sharpest tool in the shed but he's not retarded as you see people peg him to be. He's oblivious to love and has many dumb moments but people only talk about the dumb moments and not the moments where he actually stands up for himself and makes smart comments.
His insensitive nature is also another thing that people mention. Then again being the cause of Yanase's breakdown will leave a negative impression on his character despite Chiaki wanting the best for his friend (at least at the beginning before the drama happened).
Wangst: Kisa seems to do this a lot especially in episode 20 where the entire episode is him debating whether or not Yukina really loves him or not, but given hispersonality, it's natural that he would be depressed.
The Woobie: One cannot help but feel sorry for Yanase especially given these situations. He's on the short end of the stick of he love triangle of Hatori/Chiaki, he lost to Hatori because he didn't spill out his feelings sooner and when he tries to confess his feelings to Chiaki, he gets rejected twice. By the end of episode 16 he's on the verge of breaking down and you can't help but pity him.
Chiaki would also be one if only because he's so oblivious and innocent compared to the other characters. And Hatori raping him in the novel gives him some more sympathy points if he didn't feel sorry for Hatori instead of blaming him.