Since most dogs are shorter than you, it's a natural tendency for them to jump up to see you. The main reason that most dogs jump up is simply out of excitement. It's an enthusiastic greeting, reserved for times when adrenaline is running high and the dog is happy about something.
It is best all round to train your dog not to jump up at people. Not
everybody likes dogs and it can be very frightening for a dog fearful person to
have a dog jumping up towards their face. If you don't mind your dog
jumping on you, then teach him to jump on you only when it's okay and you've
given him a command to do so.
It's all to easy to inadvertently encourage jumping behaviour from puppyhood. When a small puppy comes running up to us, all wiggling with excitement and making those clumsy puppy leaps at our knees, it's too easy to instinctively bend down and make a fuss of him. What we've effectively done in this situation is to reward the puppy for jumping up at us by giving him lots of fuss and attention. Puppies learn this lesson very fast - jumping up is a good thing because he gets lots of love and praise.
Whilst we may see these little puppy leaps as sweet, your dog will not understand the difference between the jumping up as a cute puppy and the jumping up when he's a big, heavy adult! To him, a greeting is a greeting no matter what size he is and just because he's a lot bigger, he'll not see any reason to stop jumping up - even though he's nearly knocking you on your back!
To stop your dog jumping up at you in greeting, he needs to be taught from day one that it's not acceptable. It's going to take a lot of time and consistency from you to teach him this and how you react to his initial puppy leaps is going to play a big role in whether he repeats the behaviour. It is therefore imperative that you are consistent - you can't let him jump up sometimes or on some people and expect him to know that he can't do it to certain people or other times. He's a dog and if he's allowed to perform a certain behaviour sometimes, he'll think that it's acceptable to perform this behaviour all the time because he doesn't know any better.
The simplest solutions are often the best solutions - and the simplest solution to stopping your dog jumping up at you is to ignore him. If he is getting no positive response to jumping up, then he'll eventually stop doing it. All you need to do is give him the cold shoulder. Do not give him any attention whatsoever when he jumps up. Don't even push him away or tell him no because this is still attention, even if it's negative attention.
And how do we do this? Whenever your dog runs at you and jumps up, do the following.
You might find that he'll be persistent and come round to the front of you and try jumping up again. Once again, turn away from him. Since dogs understand body language a lot better than the spoken word, you're using your body to show him that his behaviour is not acceptable.
Don't confuse ignoring the behaviour with ignoring the dog. You're not actually ignoring the behaviour by pretending it's not happening - you're ignoring your dog for carrying out an unwanted behaviour. You are actually still reacting - but not in a physical or verbal way. You are reacting by actively ignoring the dog for behaving in a manner that you don't find acceptable.
This method of giving your dog the cold shoulder is an excellent way of communicating to your dog that you are not happy with his behaviour. He will catch on very quickly that he is behaving in a way that gets him no attention whatsoever and he will calm down very quickly. All a dog wants is his owners attention and love, so he will soon work out what gets him attention and what doesn't.
When is the time to praise your dog when teaching him not to jump up? When all four paws are on the floor. The instant all four paws are on the floor, you can praise him and treat him as much as you like.
Don't think that by praising him instantly all four are on the floor that you're praising him for jumping. Your dog only associates your reaction with whatever he was doing at the exact moment you gave him attention. Timing is all important and it is perfectly okay to be enthusiastic with praise and rewards the very second that his paws touch the ground - even if you were ignoring him a second before-hand.