Why do people who work on the trading floor yell so much?

"Information is moving so quickly and stocks are trading all day long," Maria says. "Let me give you a quick analogy. You're at home, you call your broker at [UBS] PaineWebber and you say … 'I want to buy IBM but I'm not paying any more than $70 a share.' He's got to get over to that post before that stock changes in price. … He's physically running over because if somebody gets in front of you and they want to buy a million shares, the next thing you know, it's $72 [per share]. Then he's got to call you and say, 'Guess what, Oprah? I couldn't buy it at $70. You paid $75.' And you're going to say, 'You're fired.'"