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.
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An anxious or playful pup may leap high quickly and suddenly “poke” at your face with their nose. That can be triggered by leaning over the top of them especially when they’re in a high-arousal situation like a homecoming or around other dogs. It may be a way for stressed pups to relieve their anxiety, so be aware of situations that cause these behaviors. Dogs control each others' movement with their body language. Think how a Border Collie makes sheep move just by getting close. You can stop your pup’s jumps by stepping close to him just before he leaps. Cross your arms and step into the pup’s personal space before he crouches to leap.
However, when we are standing or sitting in a chair and they come bounding over to greet us, jumping up on us and trying to reach our face, we must not bend over and exchange hugs and kisses. If we do we are training and rewarding the puppy for jumping up. Eventually, most of us decide we no longer like our puppy jumping up to say hello - usually when the puppy gets bigger and more rambunctious. What used to be cute is now obnoxious and even dangerous if the puppy is jumping up on children or the elderly.