To say hello, of course. Puppies are social creatures and happy and excited to greet just about anyone! Jumping, leaping and bouncing are ways your dog shows affection and receives attention. When a puppy is very young, we usually sit on the floor, let them wiggle into our laps and allow them to lick and nuzzle up close to our face. This is OK because we are down at their level.
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.