How can you overdue it, you ask. Isn't dialog a good way to move the story forward and reveal information naturally? Isn't it one way of "showing, not telling"? Dialog can accomplish those things, but dialog can also slow a story down, especially when the author relies on it to reveal everything or tries to make it so realistic that she includes every line a character might say under the circumstances without summarizing.
Here's a typical example of what I read in many submissions.
"I'm sorry to tell you," the doctor said, "but your mother's cancer has spread to her liver."
Sam couldn't answer immediately, then he asked, "How much time does she have?"
"I hate to give timetables. It depends on a lot of things."
"Weeks? Months? Years?"
"Well, certainly not years. Perhaps months."
"That's hard to say."
"I understand, but my mother has always wanted to travel to Europe? I'm wondering, would she have time?"
Of course, this would continue on even longer, but at the risk of losing some readers, I'll cut it short and hope that you see my point. This may well be a very accurate reconstruction of how this dialog would unravel. It also reveals significant information that moves the story forward. Only it moves at a snail's pace.
Consider this instead.
When the doctor told him his mother's cancer had spread to her liver, Sam found himself unable to speak at first. Then he asked, "How long does she have?"
"I hate to give timetables," the doctor said. "It depends on a lot of things."
But Sam needed to know. Would it be weeks? Months? His mother had always wanted to see Europe. Would there be time to fit that in?
Can you see the difference? First, we were able to cut eight separate line/paragraphs down to just three. More important, we've made it easier for the reader to stay with us without losing interest, and finally, we homed in on what will probably turn out to be the main conflict of the story, whether or not Sam's mother will get to Europe before she dies.
Here's another problem with too much dialog, it makes things like description, setting, time of day or year, difficult to incorporate into the story without sounding forced or unnatural. Most writers know enough not to fall back on contrived dialog like, "My your deep blue eyes are sparkling tonight" or "Shall we leave the bar and walk out onto the streets of Los Angeles?"or "By the way the sun is slanting across that field, I'd say it must be late afternoon." To avoid that, they often don't include these aspects in their work at all, causing it to feel shallow and ungrounded.
Another tendency I'm seeing, is starting a piece with a line of dialog. We are often encouraged to begin the story in medias res, but only rarely does a line of dialog, especially realistic dialog, provide the necessary impact. Take the example of the doctor. Unless the point is to expose his lack of bedside manner, then realistically you will have him preface his bad news with, "I'm sorry," and those words dilute the impact for the reader just as they were meant to dilute the impact for the receiver of the news. When we change it to When the doctor told him his mother's cancer had spread to her liver, Sam found himself unable to speak at first, the reader feels the shock of the words and Sam's reaction to them.
Here's an exercise to try. Take one of your stories that makes heavy use of dialog and try re-writing it without any dialog at all. Let it rest a day or two, then go back and try putting in dialog only where it will clearly save words or move the story forward.
Dialog can help a story along, but be careful not to overdue it.