Because discipline is like any other skill, it gets built up over time. If they're being disruptive then I stop them, but if the rest of the class can focus on me and ignore them, then I ignore them, too. If I call them out on it over and over again, then it gives them more attention and takes away from the rest of the class.
I have a fellow student who is a 2nd degree black belt right now. When he was a colored belt, he was an obnoxious little punk. He only wanted to participate in specific things and never wanted to do anything he thought he was too old for. We'd count during stretches, and it was a fight to get him to count with the group and he refused to do it by himself (because counting isn't fighting). He didn't really care how his forms looked. Whenever we'd do a game or obstacle course, instead of teaching fighting, he'd sit out and refuse to participate.
Now, he's a great guy. He's still a little stubborn at times, but he does good forms, has great technique, and he's become a stellar leader. He's gone from someone that I used to loathe having in class (and who returned that sentiment), to someone who I look forward to training with and miss when he's gone. He's stellar with the kids, he's so much more patient with them than he used to be. He's great at sparring (no surprise, since that's what he really wanted to do). We had a photographer come in to take pictures this week, and he and I were partnered up for the photo shoot.
If I had given up on this kid like I wanted to, he wouldn't be turning into the young man that he is today.