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.
3. Because they have been rewarded for this behavior in the past, whether purposely or unknowingly. Some owners may, at one point or another, allowed the dog to jump on them or on their furniture, welcoming their dog's enthusiasm to see them. Other owners inadvertently reward their dogs jumping, simply by doing something good--such as walking or feeding the dog--right after the dog has jumped. Even if they told off the dog for jumping, by providing the dog with a walk or with food, they make the dog connect these positive actions, with a not-so-positive action: jumping! The dog sees thing in a very simple, linear fashion: if he jumps, he'll get to go for a walk--or even better, he'll get fed!
Paying attention to a dog only when all four paws are on the floor can work well, if jumping isn’t well entrenched and if everybody who deals with the dog follows the rules. Unfortunately, much of humanity will get busy undermining you. “I don’t mind your puppy jumping up,” they say, while you tear your hair out. Or they get all disciplinarian, maybe kneeing the dog in the chest. That is not only mean but counterproductive, because dogs often respond by trying to appease. Since humans are usually taller than dogs, reaching our muzzles to lick them involves jumping up.