Why Dogs Jump
Like so many of the things that dogs do that people find unacceptable, jumping up is a normal part of canine communication and interaction. From the time they are identifying as dogs at three weeks old they use jumping up as a way to solicit things they want, and as a way to play and establish rank.
What Did You Bring Me?
If you've ever seen a nature show about any type of wild canines, like wolves, coyotes, or feral dogs, you've seen all the pack members who stayed behind swarm the returning hunters, jumping up and licking the corners of their mouths. It looks like a kiss hello, but what they're actually doing is soliciting a regurgitated meal. Sounds pretty icky to people, but it's another one of those this-is-what-separates-us-from-them things and is an innate part of being a dog.
Since your dog never stops being a dog, this is part of his natural greeting behavior with his people, the returning hunters who provide his meals. Not that he's necessarily expecting you to give him a snack every time you walk through the door, but it never hurts to try, at least in his mind.
However, just because it's part of his nature doesn't mean you have to go along with it. He will be happy to get your attention instead of a meal, so your job is to give him attention for what you like — four on the floor or sitting — rather than for jumping up.
What's the best way to get my dog to stop jumping up on people?
It really depends on you and your dog. Any anti-jumping program begins with managing your dog so he can't jump on people. Also add training a replacement behavior for jumping. Some dogs may also require a correction or punishment if the behavior is a well-rehearsed habit.
It's Not Just Your Dog's Problem
It starts innocently enough. Your puppy is little, or she looks so cute, or you're not wearing work clothes, so you pet her and play with her sometimes when she has her feet on you. Or somebody else, a visitor, friend, or neighbor, does. It only becomes a problem when she gets too big, or she does it to the wrong person, or everybody, or when you are wearing nice clothes.
For her, jumping up is always worth a try, because sometimes it results in good things, like petting and play. Even if something good doesn't happen, at least something happens, like some yelling or playful (to her, at least) pushing, adding a little excitement to an otherwise dull day. Pretty soon, she's jumping on everybody because she can always count on something interesting, and sometimes fun, happening.