Life is like riding a bicycle – in order to keep your balance, you must keep moving.
Stories, too, are like riding bicycles. If the story stops too long, if the forward momentum ceases as the protagonist makes witty asides, or the scenery is discussed in loving detail, or anything else unimportant to the story itself makes a roadblock, we fall over. We are no longer riding the story, um, bike, but falling out of the story–off the bike–and saying ‘Ouch!’ After which, we may or may not get back onto the bike. That particular story may not draw me back in after it’s lost my attention. No matter how lovely the details, or fine the prose, if they don’t further the plot in some way–murder those darlings to save the story.