The Power the Dark Lords Knows Not

According to the Wikipedia, the Ouroboros is an ancient symbol depicting a serpent swallowing its own tail and forming a circle. It often represents self-reflexivity or cyclicality, especially in the sense of something constantly recreating itself, the eternal return, and other things perceived as cycles that begin anew as soon as they end (especially negative or "vicious" cycles). The symbol has appeared in many religions and cultures over the last ~3600 years of human history, and has a special significance in alchemy. Swiss psychologist Carl Jung writes:

"Ouroboros has been said to have a meaning of infinity or wholeness. In the age-old image of the ouroboros lies the thought of devouring oneself and turning oneself into a circulatory process.... The ouroboros is a dramatic symbol for the integration and assimilation of the opposite, i.e. of the shadow. This 'feedback' process is at the same time a symbol of immortality, since it is said of the ouroboros that he slays himself and brings himself to life.... He symbolizes the One, who proceeds from the clash of opposites...."

Voldemort went to Godric's Hollow in 1981 to kill the son of those who had thrice defied him, because it was foretold that this baby would one day have the power to vanquish him. However in doing so, he accidentally destroyed his own body while transferring some of his powers into another body; he replicated himself in baby Harry, to some extent.

This is not to suggest that I think Harry is a Horcrux. Harry was just an orphaned, dark-haired, half-blooded wizard boy who could speak to snakes. The cycle that began with the infant Tom Riddle was poised to begin anew with the infant Harry Potter. But Harry had the power the Dark Lord knew not: love. The fact that his mother died to save him, and not in spite of him, left him with a lingering protection in his blood.

Dumbledore decided that, since Lily Potter had invoked an ancient magic as Voldemort shed her blood (figuratively, since the Avada Kedavra doesn't usually make people bleed), the bond of blood was the strongest shield he could give Harry.

Now, what was noteworthy about Lily's blood? It was Muggle blood. Her Muggle blood, her sister Petunia, is what has saved Harry.

When Dumbledore went to the orphanage to collect Tom Riddle, he discovered the matron and the other children were all afraid of him, because strange things seemed to happen when he was around, especially if he was angry or upset. Riddle had developed a taste for instilling fear in people and manipulating them using his strange powers, and this was something he never grew out of.

Was Dumbledore to let Harry go to an orphanage? Aunt Marge announces in PoA that that's exactly where he would have gone if he'd been dumped on her doorstep. Luckily, Aunt Petunia was willing to take him. This meant that Harry grew up with adults who knew he was a wizard, and despised him for it. He was punished every time strange things happened around him, he was never given the opportunity to gain power over people and manipulate them by instilling fear.

What if Harry had gone to live with one of the many wizarding families who would have been honored and delighted to raise him as a son? What if he had grown up a pampered little prince, knowing he was famous? Would he have had the same drive to destroy Voldemort for having killed his parents? Would he have had the same sense of independence and ability to think for himself? Would he have had the same intuitive understanding of what it felt like to return to a loveless Muggle environment every summer, like Riddle had to?

Harry has walked in Voldemort's footsteps, entered Voldemort's mind, faced Voldemort repeatedly on his own. If Dumbledore had not let Harry do all this, even if he could have protected him for sixteen years, Harry would have been toast the moment he came of age.

Basically, what I'm trying to say is that Harry's ability to love is what has kept Voldemort from possessing his body and mind, together with the Muggle blood that kept him from turning into another Voldemort himself. He has spent his whole life preparing to meet his destiny. Dumbledore was thinking very long-term when he first arrived on Privet Drive; that must have been what the funny pocket watch with the planets was all about.