They do it for varies of reason.
1. Like being a dick
2. Like to screw around
3. Like to upset other gamer
4. Like to see what sort of entertainment they get out
5. Like to know what tick people off
6. Having a bad day (really bad day)
7. A kid who doesn't know what he's doing
8. Don't like how other player play
9.... the list sort of goes on.
Majority of the time, a lot of gamers are immature and very short temper. They lose they cool instantly - show how immature they are, even adult - and just do whatever they can to ruin the play for other players. My advise, make some good friends, and play with them only.
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