1. To assert dominance. Whether it's jumping on people or jumping on furniture, both instances illustrate a dog that wants to raise his station in life. By jumping on you or jumping on the couch, he's saying that he's the pack leader--something that you DON'T want to happen, as a dominant dog is much more difficult to train (and can become quite aggressive). In either case, you'll definitely want to institute the proper alpha dog training techniques, if you haven't already.
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!
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.
While the idea of "training" may seem daunting, it is simply teaching an animal what behavior works or doesn't work through resulting consequences — so whether you realize it or not, you are actually training your dog every moment of the day. Fortunately for you, preventing jumping is possible even without structured training. There are some simple solutions to this problem that require minimal effort and fit easily into your normal interactions with your dog.
Our inconsistency perpetuates the problem. Some of the time we tolerate the jumping and ignore it. Other times we reward the behavior by exchanging enthusiastic greetings. But when we're dressed up and the puppy's paws are muddy, it's a different story. Reprimanding the puppy for jumping up usually does not work. Either the puppy misunderstands the reprimand as praise or he gets even more excited and the jumping gets worse. If the reprimand is severe enough, the puppy may stop jumping at that moment but it doesn't solve the problem altogether; and it certainly is not a very nice thing to do. It's very similar to a person approaching you with a big smile, arm extended to exchange a hand-shake and you bopping the person in the nose. Even if your puppy learns that jumping up on you is not a good idea, he will usually get away with jumping up on everyone else.
Understand the dog's behavior. In a dog's world, it is instinctual to greet another dog by coming nose-to-nose with them. This allows them to sniff each other's faces and become acquainted with the other dog's scent. Of course, your nose is a bit higher to reach so it is only natural that your dog will jump up to get closer to your face when greeting you. This can be quite annoying and unnecessary, but fortunately, the habit can be broken.
For pups that ambush you and bite your rear end while playing outside, hide a toy or two in the backyard and ask them to find the toy. Bad weather can give puppies cabin fever when they don’t have adequate time outside to run off the energy. Mental stimulation can wear them out, too. Show your puppy a favorite toy and then roll it up inside an old towel and knot it to make a puzzle. Encourage the pup to unravel and get the toy. You can even tie the first toy-in-the-towel inside a second one for more of a challenge to relieve boredom.
If a dog is stressed or afraid, their demeanor and typical behaviors will change. If you see this in your dog (or really any dog you know) you should step back and check on your dog. Are they hurt? Did a new dog come around they don’t know? Has the weather changed? Is it hot outside? When was the last time they had a chance to go outside to pee or poop?