Bronze and Gold
The difference between bronze and gold is almost insignificant; gold has a tad better mechanics and dies a little bit less, but the real difference is their mentality, gold players are such assholes compared to bronze or silver, for example, a couple months ago, i would go into a ranked game in bronze with a jungler and i forgot smite, people would just say 'np man its ok' or 'don't worry about it man', etc or i go top with the wrong runes or whatever the situation may be.
NOW GOLD; i went rengar and forgot smite, and nobody bats an eye except a little 'lol' at the beginning of the game. Now i tell myself, ''hey, this is gold, people are smart, right? they won't play hyper agressively knowing i can't gank and will probably be way underleveled compared to everybody else in the game.'' aha, how mistaken i was, bot lane was pushing excessively without wards most of the time, and the enemy jungler was stealing my camps on top of that, so i tried my best to just farm, hopefully get to late game and see where the team could go from there, then, but starts feeding (unintentionally, they were just getting destroyed), and so the blame games begin, everybody goes against me even thought my score was one of the best, being 1/2/*.
everybody was around level 10-11-12 and i was 9, so during teamfights i tried to stay back, as any rengar would, but even more than usual, as i watched my team get slaughtered, knowing i could not do anything. i got tilted when my team told me 'fucking useless rengar' and 'rengar too heavy' followed by 'dumbass rengar do something' along with a nice 'wtf rengar you idiot prostitute' this tilted me, as it had never or rarely ever happened in silver and never in bronze. i understand that ranked is competitive play, but people can play competitively without being assholes, or at the very minimum focus on themselves instead of others, as they have no control over them.
edit; i know am i am at fault here, but no matter what happens in a game, i think respect should always be number 1 priority.
TL;DR high elo/medium elo is toxic as hell, which makes people negative, which makes even more people negative, and so it spreads.