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.
If a dog is stressed or afraid, their demeanor and typical behaviors will change. If you see this in your dog (or really any dog you know) you should step back and check on your dog. Are they hurt? Did a new dog come around they don’t know? Has the weather changed? Is it hot outside? When was the last time they had a chance to go outside to pee or poop? 
Many puppies don’t know their strength. When they jump up and you wave your arms and try to push them off, they may think it’s a game and grabs and bite harder. Tell them it hurts the same way another puppy would, with a YELP! Lay it on thick, overact and cry and sob like the pup has done major damage. Some tough dogs get the message using this. For the out-of-control grabby, ambush type of dog play, give him a taste of his own medicine and SCREAM (very loud but very short), and fall over “dead.” Don’t move and don’t say anything. Play dead for at least 15 to 20 seconds. The shock value may be enough to send a permanent message that such games stop all interaction, plus they hurt you—and playing dogs aren’t interested in hurting you and won’t want you to cry.

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.
Homecomings and departures are a prime time for jumping up because puppies want to greet you or stop you from leaving. Turning your back on some of these dogs revs them up even more, so instead, try ignoring the bad behavior. “Ignore” means you make no eye contact, say nothing and stand still like a zombie offering no reaction to silly puppy behavior.
This week’s episode is on how to teach your dog to stop jumping up and instead greet people politely. Jack writes that he and his wife have taught their young dog that he gets attention only when all four paws are on the floor. But the dog jumps up on guests. His exuberant greetings are hard on children and old people. Today I’ll explain how to teach your dog that the best way to get people to say hi is just to sit there and not jump up.