You must understand, it all began with a tortoise.
Well, in truth, it all began with a war. Lord Shen of the Fire Nation attacked the western Earth Kingdom, because he had been told that a warrior of that region would defeat him. Avatar Oogway died ending this war, and counted his life well sold. But Avatar Oogway's companion animal, a small desert tortoise, survived him - as did Master Shifu, Oogway's closest student and friend.
Shifu had had a dream shortly after Oogway's death, that the next Avatar would be born in the wake of war, and that the desert tortoise would recognize the new incarnation.
So, Shifu stayed in the Valley of Peace, which had seen the worst violence and the longest occupation. Now there were many unclaimed children of mixed parentage in the Valley, and of these Shifu took five in particular to be his students. Mantis, Viper, Monkey, Crane, and Tigress - especially Tigress - he thought that one of them might well be the reincarnation of Oogway.
But he overlooked one child, an orphan whom the noodle-maker took in as his son.
Po did not inspire anyone to thoughts of greatness, or of mixed parentage. Po appeared to be pure Earth Kingdom, from his enormous, stocky frame to his surprisingly vivid green eyes. He always had a smile, and had some talent at Firebending, but no one thought of him as anything special - including himself.
On the fifteenth anniversary of Avatar Oogway's death, Shifu arranged for a festival and a demonstration of kung fu. It would all culminate in the test he'd dreamed of years ago - the desert tortoise would be set out, and would crawl towards the true reincarnation of the Avatar.
Unbeknownst to everyone (except, of course, for the tortoise) the prescient creature escaped from its box early into the festival, and made its way v-e-e-e-r-y s-l-o-o-o-w-l-y to the outside of the Jade Palace.
When the tortoise was found to be missing, Shifu panicked and did his best to hide it. He raced outside of the palace, stopping to ask every passerby to ask if anyone had seen a tortoise. He stopped at the temporary noodle stand. There, the unattended fire was flickering away docilely, while Po was playing with the little tortoise as though they were old friends.
And that, much to Shifu's horror, was that.