Harry was Lord Voldemort's 7th unintended horcrux. When they were at the Forbidden Forest he used the killing curse, destroying the fragment of Voldemort's soul inside him, making harry's soul completely his own. Voldemort used Harry's blood in his resurrection in 1995, causing Harry to come back to life. Also there is a possibility it was because he was the master of all 3 deadly hallows. The elder wand was loyal to him and was close by and actually "sort of killed but then got over it" him. He had the stone and cloak. If you have all 3 you are apparently immortal.