Haaaaaaaaaaaaaaate! D:
Look, it's nothing personal. Love triangles can be done well. It's just that time and time again, I've seen the love triangle handled SO BADLY that I've just developed allergies, or something.
Love triangles nearly always make at least one - if not all - of the characters look incredibly indecisive or stupid, or both, and it's nearly always possible to tell what the final outcome is going to be anyway, since the creators can't seem to help emphasising which is obviously the right choice etc., etc.
Also, I absolutely despise the kind of plot where two people are in a relationship and have started to establish some sort of security in that, only for the writer to suddenly throw in a brand new character to destabilize it. It feels like it's just a way to keep the story going for a little longer. It's cheap and it's boring and I haaaate it.
When the brand new character is a girl, in a girl-boy-girl triangle, the new girl nearly always ends up being a horrible person towards the original heroine, which just furthers the dumb idea that all girls are rivals for the attention of boys and aaaaaaaaaargh
(and you tell I hate this trope yet?)