User blog comment:WarpyNeko930/Warpy's 10 Least Favorite Things Ever in No Particular Order/@comment-5429941-20161017174546/@comment-3463132-20161017175550

The same reason One Piece fans like to talk trash about Naruto and such- when you have two similar media that go for the same demographic.

Because you're choosing which one to watch- so you compare them one way or the other. And then when someone else indirectly says you chose wrong by saying they like your opposite choice better, you feel slighted, you get angry, and you respond.