I totally understand why someone would only like reading cute, feel-good kinds of manga. The thing is, if you only care about that sort of story then you already have a massive amount of yuri manga to read, there's hardly any other kind. This presents a problem for people who want different kinds of stories (like me).
And don't get me wrong! I enjoy the charming, sweet, fluff stories too, and I think this is true of many people that want more variety (though I can't speak for anyone but myself). Desiring something else is not mutually exclusive with liking what already exists.