I think the natural tendency of most writers is to write too much the first time through. This conclusion comes from thirty years of reading student essays and crossing out countless parenthetical expressions, prepositional phrases, and wordy descriptions. Somebody has to make your project say what it has to say and no more, and that somebody should be you.
Sometimes it's whole passages. A friend who edits for me is good at locating sections that don't need to be there. "Why do we need this?" she'll ask. I don't always want to admit it. Maybe it's a great passage, with good description or witty repartee. But is it necessary to the story? Well, no. She's probably right. So the passage gets moved to the end, where I let it lie while I think about it. It sits there, waiting, but I can seldom justify putting it back in. Eventually it gets deleted or moved to a "someday" file. It may work in some other piece.
Sometimes the slicing is within paragraphs or sentences. I keep repeating the "read it aloud" mantra because it works. Reading your work aloud, slowly and with your ears tuned to the phrasing, will reveal wordiness, and then you can attack it. Here's what my rough draft said: "He shoved her toward the entrance and she stepped inside. A rough push in her back sent her stumbling a few feet ahead." On an edit I left out a repetitive pronoun, an unnecessary prepositional phrase, and a detail that added nothing to the reader's understanding. Now I have this: "He shoved her toward the entrance. Inside, a rough push sent her stumbling ahead." Tighter writing; same information.
Why didn't I write it that way the first time? I can't speak for anyone else, but storytelling is my focus in the initial draft. My brain is in a hurry to get the events down, the "what happened" part. In that hurried sense, I use all sorts of syntax and constructions that I shouldn't. But that's what editing is for.
They say Mozart wrote everything perfectly the first time, hearing the music in his head and never changing a thing. It would be nice to have that talent, and maybe there are writers who do. But guys like Faulkner, who published several versions of his classic short story "The Bear" because he kept seeing things he wanted to change, and even William Cullen Bryant, who did the same with "Thanatopsis," would admit that a work of writing art is not done the first time it goes on paper.
Or the second.
Or the third ...