Plot conveniences, such as "I'm not going to tell you this important thing about the plot because when I do it will actually make a huge difference in the plot and we need to keep it going as long as possible."
Love triangles as someone else stated. They make me think the writer couldn't think of any way to make tension in the relationship so the only way it can be done is by some third wheel coming in. You can also say extreme jealousy where one of the people acts friendly with someone else and now the other is super jealous and takes everything the completely wrong way.
Killing game genre. What's the easiest way to create drama? People have to kill each other for some stupid reason. meeeeeeeeeeeeeeeeh. Though I am reading Platinum End which is kind of a killing game, but done MUCH better since it isn't even the focus most of the time and there are very few characters involved.
Two guy characters who are obviously really close with each other and have strong feelings but NEVER ACTUALLY GAY and then a girl HAS to be shoved in the middle. SERAPH OF THE END.
Manga spoiler
Literally other characters having to TELL Shinoa that she MUST BE IN LOVE WITH YUU, even though she's there like "No" and there has literally been ZERO romantic happenings between the two. Literally zero. Meanwhile Yuu and Mika keep holding each other, basically confessing their love for each other, comforting each other, being close to each other 90% of the time they're together. I love Shinoa's character, but I really just hate that part of the manga.