Love triangles make everything worse
Why are love triangles everywhere? If this trope were to be confined to a fraction of humanity's output of fictional media, sure, perhaps I could live with it. But it is ubiquitous. TV trope's page on Love Triangle has to divide examples of this trope into eleven subpages, one for every different medium (Films, Comics, Video games, Theatre ...). And every subpage is an incomplete, non-exhaustive list, yet it still has dozens of examples. Practically every piece of pop media includes this trope, one form or another. Does no one else get sick of it?
Yes, I know that tropes are tools, no trope is inherently bad, and how the trope is used is more important. The problem with love triangles is that they are often executed terribly. Even when they are done well, the trope rarely increase my enjoyment of the story.
I know this is completely subjective, but I just don't find it interesting for one person to choose between one romantic partner or another. I also have no interest in watching one character compete with another character for the affection of a third character. Overall, I find dramatic romance subplots to be quite boring. I'm willing to put up with it if the main plot is engaging enough, the romantic subplot only occupies a small fraction of the overall narrative, and the outcome is resolved reasonably. Of course, more often than not, this is not the case.
The most common defence of "love triangle fans " is that it adheres to the "Rule of Drama", an informal guideline for fictional storytelling, which states that if there is potential for dramatic conflict, then it must not be passed over. The rule is not an excuse to stuff your story with an overabundance of drama, which could break the suspension of disbelief. More importantly, there are many different kinds of conflict, it's possible to have a story filled with drama, and not a single love triangle. Furthermore, there are a lot of interesting motivations for conflict between two characters, and I'd like to see more of them explored instead of the millionth love triangle.
Then, there is the defence that love triangles are everywhere in real life, so of course they should appear frequently in works of fiction! Except, no. While love triangles aren't unusual, they are nowhere near as common IRL as they are in fiction. There are cases where fictional adaptations of real events insert a love triangle for no reason! Why does the Trojan War have to be caused by a love triangle instead of geopolitical intrigue? Despite what Romance of the Three Kingdoms may say, Lu Bu probably didn't betray Dong Zhou over a love triangle. While we're on this topic, it should be obvious that most stories do not, should not strive to be completely realistic, and being extremely realistic is not a positive quality in and of itself.
"Why don't you just let other people enjoy what they like??"
This is probably the strongest response, against which I have no concrete defence. You should remember that this is, after all, r/CharacterRants, where people with too much free time rant about fictional tropes that a majority(or a significant minority) of people enjoy immensely. I am also being slightly hyperbolic in my rant. I don't dislike every piece of fictional media that includes love triangles.
In fact, some of my favourite stories actually contain love triangles, although it is very much a case of enjoying a story in spite of serious flaws.
Additionally, in some instances, other people's enjoyment of this trope actually do interfere with my enjoyment. Discussions about romantic relationships tend to drown out discussions on practically every other topic. Love triangles just add fuel to the fire.
Don't even get me started on the ridiculousness of shipping wars.