Just like professional sports, its just an outlet for people to do something they love in public. At least MLG players don't demand huge amounts of money for what basically amounts to, well, playing a game. They genuinely seem to be doing it because they like it, and more power to them. And unlike pro sports, anyone can compete in MLG if they have the time and dedication; age, physical ability, gender, all these don't matter. And that's great.
However, the COMMUNITY it spawned is a whole 'nother matter. Just like many pro sports fans, they get WAY to into it, but without the clear divide between player and fan that, say, football has, its led to a wave of considerably undeserved dickishness. No guy in the world goes outside for a game of pick-up ball and goes home thinking he's Joe Montana, but far too many gamers see the recognition that MLGers get for basically doing the same thing they are, and think that therefore they are deserving of the same attention. Thats why you get so many players emulating and imitating MLG players, forcing the specific rules on every aspect of the games they play in an attempt to win the same recognition for themselves.
Unlike most sports, gaming doesn't have one set of clearly defined "correct" rules. Every game is different, and every person has their own way they like to play. MLG-wannabes take offense at this. They spend their time practicing and perfecting under a very specific type of circumstances, the same as their idols. The idea that someone else could be good at the game without doing it the same way as them is incomprehensible, and therefore must be cheating or unfair, completely glossing over the fact that it was they who shut themselves in the box to begin with. The MLG rules were not created because they were the "fairest" way to play, they were made because it was the fastest way, forcing confrontation in, say, FPS to the forefront and thus focusing on individual skill. It was not because "OMG TEH NEEDLER IS BORKEN".
Basically, TLDR on the second section, the MLG community has taken the act of professional gaming without regards to the spirit or point of the game in the first place. Really not so different than professional sports in that regard. Also, the unnecessary entitlement and bravado really soil the experience for those who just wish to play on a more focused and competitive level.