Okay.
As you level up, people are more and more cocky. It gets much worse when you start playing ranked and get into higher elo.
People think that 900-1500 elo is "elo hell" as there are many many BAD players. This may be true to an extent, but at around 1650 elo and above, players intentionally troll and throw games. It's REALLY bad.
You also get a lot of people who talk shit and don't speak english very well so it induces rage.