* Lily's Sacrifice: The most important factor is that Harry's mother, Lily, died protecting him. Her love for him acted as a powerful shield, deflecting the curse and leaving Voldemort weakened and unable to kill Harry. This is a key theme in the series, highlighting the power of love and sacrifice.
* Lily's Magic: While Lily was a witch, her sacrifice was not inherently magical. However, the act of dying for another, especially a child, is considered a powerful magical act in the wizarding world. It's this act of selflessness, fueled by love, that directly affected the curse.
* The Horcrux: Unbeknownst to anyone at the time, Voldemort had split his soul into seven pieces called Horcruxes, making him nearly impossible to kill. When he tried to kill Harry, a fragment of his soul, inadvertently, entered Harry. This is why Harry survived, but it also explains why he had a connection to Voldemort throughout his life.
* The Elder Wand: Although not explicitly mentioned in the first book, the Elder Wand is a powerful and complex magical object. It is strongly connected to its owner and might have recognized Voldemort's weakened state, thus refusing to carry out his command fully.
It's important to note that while the Killing Curse is considered a near-unstoppable force, it's not entirely infallible. There are instances where it has been resisted or defied in the books.
In conclusion, Harry's survival is a complex mix of factors: Lily's sacrifice, the inherent power of love, Voldemort's Horcrux creation, and the Elder Wand's unknown influence. These elements combine to create a unique and powerful story, showcasing the power of love and sacrifice in the face of immense evil.