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.
While the idea of "training" may seem daunting, it is simply teaching an animal what behavior works or doesn't work through resulting consequences — so whether you realize it or not, you are actually training your dog every moment of the day. Fortunately for you, preventing jumping is possible even without structured training. There are some simple solutions to this problem that require minimal effort and fit easily into your normal interactions with your dog.
When your dog has all four paws on the floor, give him attention and praise. If he jumps up at any point, freeze with your arms folded on your chest until he calms down. Instruct guests and all members of your household to consistently ignore jumping. If you have visitors who ignore the no-jumping rules, keep your dog on leash during the visit and gently remove him if he jumps. For anything your dog gets in life, whether it's your attention or a meal, only give the reward when all four paws are on the floor. Jumping up should end all attention and rewards, while remaining fixed on the ground garners all the pleasurable reinforcement a dog desires — treats, petting, play.
An L-footer is a great way to stop your dog from jumping over the fence if used a little differently. You’ll want to turn it upside down and use it at the top of the fence. Create the L-shape and securely fasten the short side to the top of the fence and have the rest of the hardware angled towards the yard so that when your pooch looks up they will see fencing.
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.
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.

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? 

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.
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? 
Our inconsistency perpetuates the problem. Some of the time we tolerate the jumping and ignore it. Other times we reward the behavior by exchanging enthusiastic greetings. But when we're dressed up and the puppy's paws are muddy, it's a different story. Reprimanding the puppy for jumping up usually does not work. Either the puppy misunderstands the reprimand as praise or he gets even more excited and the jumping gets worse. If the reprimand is severe enough, the puppy may stop jumping at that moment but it doesn't solve the problem altogether; and it certainly is not a very nice thing to do. It's very similar to a person approaching you with a big smile, arm extended to exchange a hand-shake and you bopping the person in the nose. Even if your puppy learns that jumping up on you is not a good idea, he will usually get away with jumping up on everyone else.
Why do dogs jump up? A plausible explanation arises from their ways of communicating with each other. A puppy greeting an adult dog often licks the adult’s muzzle -- a polite, deferential behavior. Dogs, of course, descend from wolves, among whom muzzle-licking is how pups get the grownups to regurgitate food for them. Domestic dogs rarely nourish puppies this way, but muzzle-licking has survived, maybe because deferential behaviors are handy for a social animal. Think of humans saying “No, after you.” Muzzle-licking is also an appeasement behavior -- something you trot out to de-escalate a fight. A human might lift up his hands, palms toward the person he’s arguing with.
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