My pettiest reason is ironically only applicable when I previously really liked said character. Nothing annoys me more than when a well written, intelligent character will suddenly decide to tie themself to another character. If it serves the plot or if the other character is fun, then it's fine. But some authors seem to really like attaching their main characters to some child character for no reason. Nothing is more annoying then having the rhythm of the story be interrupted because the author decided this character just needs some completely useless, often unlikable gimmick of a character. My problem is that I dislike even well written child characters unless they are masterfully done. This means that just having an important child character can be enough for me to drop a story. It's unreasonable, but I'm just weird that way.
Also, having a character that is established to be extremely smart or impressive at something be one-upped by some other character can be really annoying if set up wrong. So many stories have this trope of the MC being well established as an elite at something but having another character hate them. This other character will somehow surpass the level of the MC in like a third of the time due to pure hatred. It invalidates the MC's struggles and serves as a very cop out story beat.
Lastly, when the main character is unreasonably obsessed with another character. It doesn't even have to be a romantic relationship. So many stories will have the MC spend like a year or two with a specific character and then have the MC and character go separate ways. The MC will then proceed to spend years away from the latter, often with other companions (that are usually more likable). Despite the in-universe time spent with their new companions being far greater than the time spent with the former, the MC will constantly think about their old companion and often will have their goals be completely tied to their old companion.
After writing this, I find that most of my issues with MCs have to do with how they interact with their companions. I guess I'm happy if the main characters and companions are written well.