I could give one of the nice answers like Derek or Jar has given, but a lot of the time, I compete so I can win. I like beating people at something they're good at, and I'm good enough at shooting to beat some of the people some of the time.
I've certainly made some friends and had a fun time doing it, and I don't think I come across as a jerk or am not friendly... but I'm fundamentally pretty competitive.
Nothing wrong with being competitive. That is part of the fun!! It is awesome when you beat somebody by one percent or less! Then again... its also feels great when you open the can of WHOOPASS too!