Too many reason to fully enumerate: get out of the house on Sunday mornings, have an excuse to sequester myself in my man-cave with my 550, gives me purpose and a reason to get out and pull the trigger. The list goes on.
But, one thing I really enjoy is watching people 'eff up. I don't mean this in a sadistic or schadenfreude kind of way. High level competition in any sport is really about consistency and making fewer mistakes than they other guy. I love to see people execute a great plan, and it's interesting when they fall for traps and have brain farts.
I compete to win, or at least try. Matches are a great place to test yourself and your equipment, and a great place to learn from shooters who are more proficient with superior equipment. The crowd is great, I enjoy the company as shooting attracts a great group of people. I also really enjoy watching Knucle Dragger screw up.
Depends on the sport. High power because it helps me keep my mental discipline, IDPA because it's just plain fun, PPC to develop accuracy. All happen to be good training but frankly that's secondary to me.
I just enjoy it. Harmless fun. I grew up watching westerns and war movies/shows and was always impressed by good marksmanship; even though I was watching fiction I still liked the whole idea of being a good shot. So, when I was able to learn to shoot, I always tried to be a good shot. Participating in competitions is just a way for me to give myself a little nudge to get out there and go shooting.
A multitude of reasons. I started IDPA for the chance to practice with my carry guns in a more dynamic environment. I've stayed (and added USPSA, bullseye, and a touch of multigun) for the challenge and motivation to improve, the friends I've made, and most of all the fun.
I like the constant challenge and striving to improve. Most of all I love the heart pounding and hands shaking so bad I can barely fill a magazine feeling at the end of a great run on steel or USPSA field course.
Well... I guess the people that are around the matches are O.K. too.
I enjoy the mental challenge of the games, tinkering with the guns and ammo. I've met great folks from all over the country, and visited places that I probably would never have seen. The friends I've had the longest, are all people that I've met while shooting.
On top of that, I like to see Knuckledragger's expression when he finds out I'm shooting Production that day.
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.