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.
Dogs like and need consistency, so if you are not allowing your dog to jump on you, everyone in the family and everyone who greets the dog must do the same. You, as an owner, must make sure this happens. It will only confuse a dog if you allow them to jump on some people who say they do not mind, and tell him not to jump on others. Once you decide you do not wish your dog to jump on people you must apply this to everyone at all times unless you give the dog a command to jump. A dog should never jump on a human of its own free will.
Jumping up seems cute when your puppy is a little guy, but when he grows into an adolescent, jumping on people can become more than rude. These juvenile delinquent dogs haven’t learned how to control their excitement and can bruise, scratch or knockdown owners when they launch themselves and plow into you with their paws and claws. With small dogs and young pups, these tips to stop jumping up can work.

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. 
I remember going to a dog park where a little 4-month-old Boxer puppy ran around jumping on everyone. The dog was not heavy enough to knock most of the adults down. However, it left everyone with muddy pants and the dog was big enough to knock over small children. The owner did nothing to stop the jumping puppy. After all, it was just a small pup. Everyone around her was pretty annoyed at the muddy prints the dog was leaving all over their clothes. That is an owner who will have a problem with her dog jumping on people when it gets older.
Puppies should be told from day one, "no jumping." Anything you do not wish your cute little puppy to do when it is full-grown should not be allowed when it is a puppy. Think about the behaviors you allow your puppy to do; is it something you will always allow him to do even when he is full-grown? If the answer is no, do not allow your puppy to do it from day one.
When he gives up and stands still (staring at you like you’ve gone crazy), THAT’s when you greet your dog and give him some love and attention. This is the reward he was looking for. Following these rules will ensure he learns that he will only earn that reward by not jumping (don’t overdo the high-pitched voice saying, “GOOD BOYYYYY” or it will just rile your dog up even more to start jumping again).

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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