The problem is that using only positive reinforcement does not work on all dogs and can actually cause behaviors to become worse, or in some cases much worse.

Negative punishment is when you take away a desired stimulus after a undesired behavior is performed making the behavior less likely to happen in the future. For example you are walking your dog and they start to pull, You stop walking and wait till your dog stops pulling.

Dogs respond best to praise—and food or toys. Positive reinforcement training uses a reward for desired behaviors. Because the reward makes them more likely to repeat the behavior, positive reinforcement is one of your most powerful tools for shaping or changing your dog's behavior.

When meeting dogs, Cesar Millan always institutes the no talk, no touch, no eye contact rule. You give the dog no attention until he is in a calm, submissive state, then you can praise him and pet him as a reward for being in that state. This technique works very well but can be difficult to follow.