Children are loud and uninhibited. It's in their nature -- they're supposed to be rambunctious and noisy (to a point). A child can be sitting two feet from you and she will speak at the same volume she uses when she's playing on the soccer field. That's when you politely say, "Use your indoor voice."
Of course, even when using their indoor voice, most children don't think to filter their thoughts before they speak. How many parents lie about something ("It's a pleasure to meet your husband") only to have their children ruin everything ("You're right, Mom, he does look like my gerbil!")? Immediately after this occurrence, parents teach their children that some information should stay private.