It sure as hell isnt because the game is good. The main cause of toxic behavior is in fact, the state of gameplay.
Riot has really screwed many players over with supporting snowballing, high damage, True damage, percentage damage and dashes on nearly every champion they have made recently.
People are becoming more toxic because they know they cant come back from a defeat! The damage is so high that if the enemy gets just one item ahead of you hes got an advantage on damage and can out spam you to death in one trade.
Its also so easy to win when your team is snowballing, people are starting to troll more because of how easy it is to win, and the fun of winning is now gone from this game. Some good aspects of the game would be bots or playing flex with a friend, but its still horrible gameplay that makes many quit for the day.
Do not get distracted by riot constant production of the newest prestige skin. Stop playing if your feeling stressed. Stop paying if you feel they are doing a terrible job on this game. Stop posting if your going to come back 3 days later, because we all know how much of a drug league is.
I hope this helps you realize the bulk of riots problem and helps you quit.