Especially with the placement matches, please don't queue up if you're already in an bad mood. It can make your judgement really clouded.

Last game I was in, one person left immediately within 5 minutes, because he failed ganked top and gave opponent double kill. However, we were winning the game very hard, as mid and bot were dominating. Needless to say, we were about to lose, but still try harded.

He did eventually came back 15 - 20 minutes into the game and I had to talk some sense into him, calming him down, despite him continuing keep calling people morons.

I think he was angry and rage quit because he already lost 3 games in a row and that was his last placement match game.

Please don't queue up if you're upset about your previous losses. If you're already in a bad mood, the upcoming games will most likely be not better than your previous ones.

You win more games when you enter a game with a better mood.