There is not much objective information about why players stop playing League of Legends. Although some players explain their reasons on why they are quitting the game on the boards, many of those players don’t actually quit playing LoL for long time and therefore are not a reliable source for reasons to stop playing League.
However Riot published a little bit of information about this topic and therefore we know that toxicity is the second biggest why people quit playing other online games (i.e. not League) . Sadly we don’t know which games Riot was referring to.
We also know that players are 320% more likely to quit the game when they experience toxicity. [590,599]
590 https://ask.fm/RiotLyte/answers/131708836802 ; Page 165
599 https://ask.fm/RiotLyte/answers/131356430530 ; Page 186