Why are SOME people on this game so arrogant?

I just had a horrible exchange with a support {{champion:7}} player... She had a horrible attitude and kept going on about how she "outplayed" us. Although I did not die once and their {{champion:238}} who was a good player won them the game. I asked her after the game, "why do you have this attitude?" and she basically told me that I was "flaming" her. I do not understand why some people in this game have to behave like this. People act as if TSM will phone them tomorrow and ask them to join their team. But they fail to understand that players with a horribly arrogant and toxic attitude towards their fellow players will never get very far as pro's. Players who act that way on the pro scene, do not tend to last very long as Riot does not want arrogant people who are going to disrespect their opponent. So why do players feel the need to do it, surely they should be following the example of these professional players and playing with a respectable attitude?
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