1. To assert dominance. Whether it's jumping on people or jumping on furniture, both instances illustrate a dog that wants to raise his station in life. By jumping on you or jumping on the couch, he's saying that he's the pack leader--something that you DON'T want to happen, as a dominant dog is much more difficult to train (and can become quite aggressive). In either case, you'll definitely want to institute the proper alpha dog training techniques, if you haven't already.
In order to perfect the proper greeting routine, your dog needs much more practice than that. You can speed up the training process by leaving through the back door and returning through the front door over and over again. When your friends come over, have them do the same. Each time, ask your dog to sit-stay before opening the door. At first his excitement will make it difficult for him to concentrate but after you've repeated this process 10 times, he will calm down and be able to concentrate. Before asking your dog to sit-stay in this distracting and exciting situation, be sure he has a reliable sit-stay in normal, non-stressful situations. And of course you can always just hold the dog in a sit if need be.
Determining if this is your dog is a little more complicated, but it is often seen when your dog isn’t very excited (or that there isn’t much of a reason to be excited). This is more than likely to happen when you are in their way or have something that they want. They will usually jump on you and try to push you back more than just jumping up to give you kisses. Often times they will jump on you and try to hold on when you push them away or try to move into them. Sometimes moving into them may elicit a growl because they want to back you off.
Make sure other family members and visitors understand this command, as well. It is crucial that your dog understand that all family and visitors are above his rank in the pack. Otherwise, he will lash out and try to assert himself on everyone. While using the Off command may not be appreciated by most visitors, but it is more important that a dog has a good greeting manner with all people. A little effort on everyone's side makes your dog disciplined and docile.
Paying attention to a dog only when all four paws are on the floor can work well, if jumping isn’t well entrenched and if everybody who deals with the dog follows the rules. Unfortunately, much of humanity will get busy undermining you. “I don’t mind your puppy jumping up,” they say, while you tear your hair out. Or they get all disciplinarian, maybe kneeing the dog in the chest. That is not only mean but counterproductive, because dogs often respond by trying to appease. Since humans are usually taller than dogs, reaching our muzzles to lick them involves jumping up.