Great trainers don't just run drills or take their dogs into a field and let them chase birds. Great trainers start each session with a goal and specific task to accomplish. They set up drills and scenarios that help teach the dog bits and pieces of a larger concept. By micro-focusing on areas that might prove problematic to the dog, they can anticipate trouble and administer well-timed corrections, praise or avoid the issue altogether.
If you're not anticipating how your dog is going to behave to a situation, you're not really training; you're just reacting. If your dog makes a mistake because you didn't anticipate the problem, you're effectively teaching him to do it wrong. To train after reaching that point requires that you correct the dog to teach him that's not what you wanted.
Sometimes negative reinforcement is the way to go and what is required, but to wholly rely on it is not only lazy, it's unfair to your dog.
With a balance of positive and negative reinforcement properly administered, you can teach your dog how to react to very complex scenarios. And, as George Hickox and Dan Irhke both pointed out at a ...