Hilarious, the toxic posters here, Big Piece and Mushroom Midget, are correct, they just put it in a poor whiny voice.
League is a tense game. You have to stay for up to 60 minutes. You have to rely on your team, who are not always the best, and you can't leave. You can be countered in lane, you can meet a one trick Riven main. One of your teammates can be having a bad day. And if your internet goes out, you get a punishment, which while understandable can be annoying.
All these things make League a rough game. And the reason why it is so important to have toxicity as low as possible.
Always take a break after a couple bad games of League.