Nobody likes to have a handful of groceries knocked out of their hand or get muddy paw prints on their new pants as they walk through their own front door, let alone someone else's front door. Jumping up can be a cute greeting when you have a small puppy or dog at home, but as time goes on, it can become quite annoying to you and your guests. Teaching your dog to calmly greet you and your guests, without jumping, will create a much more enjoyable environment to enter into after a long day at work or when receiving guests at home.
When a dog jumps on a human of its own free will, it is not "greeting" the human, it is asserting its dominance over the human; it is the dog communicating that it is alpha and/or wishes to own/control the human. A subordinate would never dream of running over and jumping on the alpha dog of the pack. Space is respect and lower members of the pack respect the higher members. If your dog jumps on humans, it does not respect them. Note: when a young puppy jumps on humans it is sometimes its attempt to reach one's face. Puppies need to be taught not to jump up on humans as this behavior will manifest into other meanings as the puppy grows up into an adult dog.
4. Use the "Sit" command. This is one of the most basic, and important, commands. As such, if your dog is well trained, he will respond to the Sit command before even thinking to jump. If he does not respond well to the Sit command, then more training is necessary, as this is the foundation for dog obedience training. Train him until he responds to it in everyday situations, such as greeting and going for walks. The better he understands the Sit command, the much less likely he is to ever jump at all.
Unless given a command to jump, a dog should never be allowed to jump on humans. A dog that jumps on humans of its own free will is a dog that does not respect the person it is jumping on. Even those cute little toy dogs should not be allowed to jump on people. While you may think it is cute, it is not cute to everyone else. Not only is it annoying to most people to have someone else's dog jump on them, it can also be dangerous. A jumping dog can knock people over, muddy their clothes, put runners in nylons and scratch the skin.