Sometimes I'm guilty of this and it frustrates me to no end. This is what happens: I'm usually away from the office or without my trusty laptop. An idea comes to mind. I spin it around and begin tinkering with it. Characters appear and I imagine the structure, the shape, the form of the story. I begin to write scenes in my mind, playing around with plot and other discourse elements. Soon, the story has become complicated and practically done in my head. The only thing left is to actually set it down. So when I get time with my laptop, I attempt to write it down - and either get irritated with the story (because I am reduced to transcribing something, yes, even if I "wrote" it) or I get bored with it (because the story no longer holds any discoveries or surprises, which are part of the creative process).
I end up with a polished first paragraph and not much else. Dialogue becomes stilted (because I cannot recapture the sound and flow of what I envisioned and heard earlier) and it becomes an exercise in futility. When this occurs, the writing time is spoilt and the story simply cannot be written.
What I do is walk away from the story and start something new. Often, this works for me; I am able to write something impromptu and take joy in the discovery of characters and what they end up saying and doing.
I'm not saying "Do not plan out your story". But I find that it's better to leave some space for your story to breathe, to explore the lacuna and odd bits that are not so easily rationalized or ironed out.
There are times when I'm sitting in front of my laptop, happily typing away, when I focus on a plot point or something and end up overthinking things. When this happens, I end up with something "precious", something that sounds contrived and oh-so obviously structured. I believe in structure, of course, but normally prefer that it does not draw attention to itself (I've written stories based on odd structures that do draw attention to themselves, but these are deliberate).
If I could write in the following way most of the time, I'd be a happy man: start with an idea and get "into the zone", just writing, not editing for grammar or content or style (because all of that can be done in the editing pass). When this happens, I write for hours without stopping (and, sadly, growl at any noise or disturbance generated by wife or daughter). The only time I stop is when I become aware that I'm writing crap or when my shoulders ache or when my fingers cannot reasonably catch up with my thoughts. When I read over what I've written in this mode, I'm usually happy (though, I must confess, also sometimes baffled by what the hell I was trying to do).
Failure to prepare is preparation for failure - but only to a certain degree, in terms of writing. I must have something to write about (Charlson Ong told me that there's no such thing as a writer's block, only a lack of subject matter), a sense of where the story wants to go and a possible structure.
The rest of the process is discovery.