This contains massive spoilers for a One piece (water 7 arc), Bleach (arancar arc), Naruto (i forget where, after the kage meeting?), History's Strongest Diciple Kenichi , Shin Angyo Onshi.
I keep coming across a certain scenario that pisses me off, its used way too often. Girls sacrificing themselves or joining the other side for some mighty retarded reason.
One Piece:
Robin goes over to CP9 to "save" the Strawhats. Stupid
Bleach:
Orihime goes over to Aizen's side to "save" Ichigo and co. Stupid
Naruto:
Sakura goes to Sasuke and tries to act like she's joining his side to kill him. Not as stupid but still pretty dumb doing it by herself.
Shin Angyo Onshi:
That girl (the main one whatever she's called) goes over to the other side, after believing exactly what the evil guy says with no proof. Just retarded.
History's Strongest Diciple Kenichi:
Miu goes with that guy to find out about her father (she probably does it again and goes all the way to the bad side later but I haven't read it all so IDK)
There are probably more but I don't want to list them all.
But in the end we know whats going to happen, no matter how they feel in the end they will be saved by the people they betrayed and go to the good again after realising how stupid they are, and they will be forgiven even though all they did was give the enemy the advantage. Maybe its because these are shounen manga and boys like to save girls but its irritating how stupid they're reasons are for doing it and how they never think "Hey maybe my friends won't like it if I go over to the bad side to save them." and why would you believe the evil side in the first place? Whatever they say is likely going to be a lie anyway.