The reason dogs jump up towards or onto people is usually because they are excited to see them and they are trying to interact at a closer level than they can from the floor.
Dogs greet each other face to face, and for this reason the dog is trying to see the person's face more clearly, and possibly wishes to lick the face which can be a sign of affection or submissiveness.
Jumping up and leaping around is the way a canine shows excitement. The behaviour is usually learned from when they were a puppy.
Puppies are such cute balls of fluff that we tend to pick them up a lot and let them lick our faces. Every single time we do this the puppy learns that it is acceptable to be up close to a humans face and they can come to depend on this proximity either to interact or to submit in the case of a more submissive dog. We are unwittingly training them to jump up on unsuspecting humans!
As that cute ball of fluff grows into a big strong dog, we do not find the behaviour quite so appealing. Jumping up on a human visitor is just a follow-on action from the behaviour learned as a puppy.
Most visitors are not happy to have a 50kg dog jumping up on them and knocking over their children. Ironically the owner or visitor does not react in the way the dog anticipates and the dog is disciplined. This can lead to a confused and maybe even nervous dog. Suddenly the dog has a behavioural issue and the owner has a problem.
The simple way to avoid this is to set the ground rules from the start and never allow the dog to jump up on anyone…with no exceptions even when only 8 weeks old!
You can lower yourself to the dog when it suits YOU, but the dog must wait until this happens, he should not demand (and usually succeed) in blackmailing you for your attention by jumping on you, this is only going to lead to worse behaviour problems later and you will therefore be doing yourself and your dog a big favour by nipping this behaviour in the bud as early as possible.