I'm gonna necro this just to say that one's attitude/mentality in league is a reflection of a lot of life-skills (or lack thereof) and can help you improve on those things.
for example: patience, awareness of emotions, mental fortitude, frustration, management of emotions, tolerance, positivity, mental attitude.
These are things that can be improved upon by checking in with yourself and having the courage to stand up to your own feelings during a match. These are real feelings and they are not lessened by the fact that it is "just a game".
Every match is a way to practice these skills and is a way that you can pull yourself up by your own bootstraps to become an even better person than the shit-tier rager you were when you started playing (speaking for myself, and this isn't to say I don't still get frustrated or tilt). However, I have noticed that making the conscious decision to not be negative or flame, or even to be the pillar of mental fortitude and positive encouragement that your team of ragers NEEDS can make the difference between winning and losing a game.
If you're positive it helps to foster a greater sense of teamwork and alliance between your teammates. If you're negative it breaks any alliance you and your teammates may have and breeds spite, petty revenge and other actions/attitudes which lead to the downfall of a match.
Also, someone who isn't me has 4 perma'd accounts. I have heard of individuals with many many more perma'd accounts than the individual I'm referencing. I think that those people who have 20+ banned accounts and who are ~diamond are good enough at the game to win a majority of low-elo games to get to diamond, but then their mentahl screws them over and they become hard stuck. They limit their own improvement by being negative.
tl;dr league can help you overcome yourself and be a better person, but it's just as hard as any other method, and requires the courage to face your own negative emotions.