Why are people so rude?

Tweet

I was playing a multi-player online game earlier and out of 9 players at least 4 coutries were represented. One person (I don't think the country is important so I won't mention it) was trying to start a conversation, the conversation was fine, but 1 comment was made about his country, then another comment and so on. I, being a mom, diffused the situation, but even after this person should have left the game (he lost had no reason to stay) they stayed and continued the conversation, which had turned to trying to start an arguement. Again, I told the person they were being rude and this was a friendly game, but they persisted. Why can't people just be nice to each other? Especially in a place where everyone there is just trying to have a good time.