do people just not want to get carried?

I just don't know maybe i just get mad when it's a game that you completely have the upper hand and someone does something so stupid that it throws the game out the window. I try to stay positive and i am because i want to win and work as a team but it's the only thing that gets my nerves ex. the adc gets caught in river for no reason at 50 minutes losing a game where everyone is winning in every way on your team. something like that just pisses me off to no end at how people make such an obvious and stupid mistake and single handedly throw the entire game. It, imo, is the most frustrating part of league of legends.