What is the main reason you lose games?

Greetings fellow summoners! There could be a plethora of reasons why people may lose a game, ranging from bad match-ups all the way to downright terrible team mates. Winning streaks are always nice and losing streaks.....demotivating, but we can't possibly win ALL our games. So I'm asking you..... What is the main thing that causes you to lose games? Have you ever caused your team lose a match from your mistakes?
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