Some people like me can get very angry and emotional at bad manner in competitive games. The absolute worst thing in the universe you can ever do is of course write gg first when they win, but there are also much-less-bad-but-still-annoying things opponents can do like insulting your skill when you make a mistake, posting a smiley face when they're about to have an advantage, lecturing you about you what you did wrong when you make a mistake, etc.
What almost all of these people (these being people the ones in my reference class, who are hurt by this stuff) don't realize is that they have a superpower to fight back. E.g., I sometimes watch
Artosis play starcraft. He plays the hardest race and often gets angry at PROTOSS, who are no doubt much easier to play, especially if you maximize for MMR rather than skill, as most people do. It's all very relatable. However, he makes the mistake of displaying his true emotions to his opponents. When he feels like his opponents won unfairly, he doesn't gg, he just ragequites. This happens in over 50% of games. Sometimes, he first informs then in the chat that they're subhuman beings with varying levels of subtlety and then leaves.
This is the totally wrong way to do it. That is how you
lose. That is how you let your opponents win.
The secret is that [the importance of writing gg yourself before you leave] is positively correlated with bad manner of your opponent. If you play absolute subhuman scum who actively delights in making people feel miserable, especially if they also cheat via streamsnipe or something comparable, that is the time where it is the most crucial to write gg before you leave. Because that way, and *only* that way, can you avoid the real defeat even when you lost in the game. The moment you write gg, all of their glee disappears because they realize that you haven't actually been hurt. They think they have made you miserable, they think you are about to get angry and shout and them, but then you write gg as if nothing happened, and within a second, all of that disappears. Seems like you're not even upset. Huh. Maybe next game.
This is the superpower that most people like me don't realize they have. If you know this, and if you can do it consistently, then you never truly lose. Someone may insult you, write offensive gg, make fun of your skill, but the moment you write a good-mannered gg before you quit, they know none of it worked.