When your dog was a puppy, he was much smaller and when he came running to you he would jump on your ankle or maybe your knee, no big deal right? And in response to your little bundle of joy, you would reach down and pet him, maybe even pick him up.
You have just taught your puppy a very important lesson, "when I jump up on my owners leg he/she gives me love!" Fast foreword a few months and now that once small little puppy is 3, 4, maybe 5 times the size he once was. He doesn't understand the difference, he just wants the love that he knows how to get by jumping on you.