There are many reasons dogs like to jump up on people. They want (and often get) attention from jumping. Dogs greet each other by smelling each other's faces. Puppies get food and attention from licking the muzzles of adult dogs. You will notice it is usually the more puppylike, excitable and submissive adult dogs who jump. Teaching them not to jump takes time and patience.
Teaching a Puppy not to Jump Up
If you have a puppy, teach it from day one that jumping isn't allowed. The most effective way is simply to ignore the puppy when it jumps. Do not look at it or say anything, and definitely don't pick it up. There's nothing wrong with picking up a puppy to cuddle it, just don't do it when the puppy is airborne, because this rewards and reinforces jumping. ...
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There are three main reasons why dogs jump