The reason some people don't like the game is because of these few reasons.
1. Their champion gets nerfed/reworked. Riot games wants everything to be fair so that everyone has a chance to win and have fun. If your champion went over their average win rates, then it gets nerfed-and people get angry because they can't "dominate" anymore. Well that just means they need to try harder for their new champion.
2. Toxicity. Yes, I can agree that League games can get very heated but the thing to remember most is that when someone gets toxic, don't get involved even if its targeted toward you- trust me, just mute them and you will do better.
3. People move up in ranks, but then it gets harder to win (because opponents get harder) so sometimes you don't win-which is totally fine but it can trip up some people.
Honestly, I love League of Legends and would highly recommend it to anyone 13+