If you're the type of anime fan who's content to just watch shows and stay off the internet, you may not know how heated shipping gets. But, if you discuss series with fellow fans online, or read up on other people's opinions, you know people care about shipping.
When fans get invested in a couple they love, they will defend their opinions vigorously. Conversely, people seriously rile themselves up when it comes to the pairings they dislike.
Some controversial ships are canon, while others never actually happen but the series offers plenty of hints. There are also some quirky, creative anime ships out there that are based on imagination, attracting fandoms full of dedicated fantasy shippers.
Does nuanced, respectful dialogue on controversial anime couples exist? Sure - but discussions often become ugly. Anon hate flies into people's inboxes, and in one case irate fans sent death threats to the creators of DARLING In The FRANXX after Ichigo kissed Hiro.
Break out the popcorn, because it's time for some serious drama.
If you're a Bleach fan who was perfectly satisfied with Ichigo's decision to pursue a relationship with Orihime, you probably know of salty Rukia fans who thought Ichigo should have chosen the Shinigami.
On the flip side, if you're into Ichigo/Rukia, you've likely encountered Orihime fans who come off as smug because their ship sailed and yours didn't.
Fans of Ichigo/Orihime cite their obvious canonical love for each other, as well as their complementary personalities. Detractors claim Orihime's personality is too subservient and that their dynamic is one-sided and uninteresting as a result.
Conversely, Ichigo/Rukia fans feel they each actively changed the other's life and their equal partnership feels more worthy of elevating to romance. Meanwhile, those who don't like the pairing claim Rukia is way too old for Ichigo (she's hundreds of years old), cite her canon relationship with Renji, and that no matter what your preference is, Orihime and Ichigo are in fact a couple.
Despite the issue being long resolved in canon, fans never forgot the conflict between the two ships.
To say the endgame pairings featured in Naruto sparked controversy puts it lightly - many people grew incensed over the final ships.
Some were annoyed about how all of the endgame pairings were straights couples and argued that Sasuke and Naruto's friendship developed more like a romance.
Others simply disliked Sakura's relationship with Sasuke, but the reasons varied: either they felt Sakura was obsessed over Sasuke beyond reason and that he deserved better, or they felt Sakura deserved better than a deadbeat husband/dad who previously tried to end her life.
Meanwhile, Naruto/Hinata garnered criticism for different reasons. Some found Naruto's relationship with Sakura or Sasuke developed much more than his with Hinata, while others argued the relationship is based off of Hinata's wish fulfillment and not Naruto's genuine interest. Some also objected to Hinata being treated like a damsel in distress whenever Naruto is around.
Not only did people disagree on endgame pairings, they also never agreed on reasoning. Unless you agree with someone on all the nuances, you're likely to get into an argument.
Even people who liked the final pairings didn't necessarily agree with each other on the pairs. If you want a Naruto pairing that isn't controversial, try Shikamaru/Temari - most people seem to like that one.
Nisekoi, a harem anime, intentionally sets up several different possible endgame pairings. It tries to make all of them appear plausible and make viewers care about them, and then ultimately chooses a single endgame pairing - in this case, the ending was Raku/Chitoge.
Naturally, the people who were invested in the other possibilities felt peeved. Those who didn't like Raku/Onodera claimed Onodera possessed no personality and never felt like a real contender in the first place. Anti Raku/Chitoge fans felt Chitoge was violent, annoying, and generally a terrible partner, chosen largely to annoy fans of the series.
Fans of each pairing felt theirs had received the most development, and the winner was bad storytelling.
The rancor between the two largest ships in the Free! fandom feels somewhat baffling. While a majority of ship disagreements start over moral issues or one ship being canon, Free! didn't have any canon pairings, and none of the popular ships posed ethical problems like incest, age gaps, or abuse.
Furthermore, little to no conflict happened between the Free! straight ships and queer ships - nearly every pairing in the Free! fandom involved two teenage boys.
Why then did RinHaru shippers and MakoHaru shippers butt heads so dramatically, even going so far as to send anon hate to people who weren't onboard their ship? There's no clear answer to this - people let their passions fly when fandom is involved, and they aren't always inclined to be polite with people who disagree.