I honestly have a smurf account that I never touch unless I'm playing with friends that truly have no or little clue on how to play the game.
(It's level 6. Woo.)
Meanwhile, other people have other reasons for smurfing.
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Most likely, the people that you're queuing with that mentality are the people that make a smurf because their original account got banned or something.
(It's much easier for people to get a fresh start than to sit through their punishment.)
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Or, people that have been losing games in a row and end up throwing a fit.
(I think people should take a break after a loss, to be honest.)
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He or she is bored with his or her current rank and wants to re-grind to hit 30 for placements.
Losing gives less exp, therefore slowing grind.
I'm really sorry for your bad experience in that one game, but you'll honestly get more games than that.
Don't let other people bother you, especially if you're only going to play with him/her for one game.
If you're really bothered, there's always a "Report" feature at the end of the game.
People say it doesn't really work, but I've honestly gotten the, "Due to recent reports, this guy's punished" (or something, I don't remember).
Lastly, if you EVER want to re-queue right after, I suggest waiting a minute after because it's also possible to re-queue with the same exact person that gave you a hard time.
Also, some people might even bash you for DOING WELL.
I remember being flamed for playing Akali and killing everyone on the other team.
That game ended with the enemy saying, "BG, you won because of broken/op champ".
So yeah.
As a Bronze 5 player, Bronze 5 isn't really a bad rank, if you ever want to rank.
Bronze is pretty much made horrible because we're categorized as bad, when in reality, it means that we have a lot to improve on.
However, more people end up blaming each other than getting along with teamwork.
TLDR;
People smurf/become mean when things don't go their way.
(Even my smurf account example. I do not want friends to die by level 30's.)
Report if necessary.
Some people are just mean.
Avoid ranked because some people are wayyy too serious about winning a game that they end up cursing someone else out.