For some of my friends, it was the realization that we are all on League just to have a good time, and belittling others (sub any other toxic behavior there) isn't a good time.
I think of League like chess. When people play chess, be they amateurs, professional, etc. they don't just go cussing people out and saying "kys" (i know, extreme, again, replace any toxic behavior) when their bishop gets taken. You might hear a "good play" or similar. But they just move on and think of their next moves.
Think of League like chess. You can't control if they take your knight/bishop/queen/tower/inhibitor/kill your teammate/your teammate has died 5 times/etc. So just move on. And only think/talk about your next play.