Death has known Harry his whole life, from that fatefull killing curse to the one in the Forbidden Forest, he's been by his side.
He sat with him while Severus Snape cradled his mother as Harry cried, He held him in the tiny cupboard after a beating too harsh, as hunger atempted to bring him closer to that final breath.
He stood behind him when he encountered Tom Riddle Jr for the first time, (the second time), He knelt before him as Fawkes cried for him, He watched him as Harry battled his minions for Sirius Black's soul and the boy's own.
He flew with him against a dragon, swam with him to the depths of a lake, He stayed with him in a graveyard as he stood before Tom Riddle Jr's insanity.
He watched over him as he crept into the Hall of Prophecies, sat with him as he mourned Sirius Black's fall, He stayed with him though his journey to find all the ways Tom Riddle Jr attempted to avoid Him.
But you cannot avoid Death, not for long, Death is inevitable, it's the end that comes to all, from the smallest single celled organism to the great trees that span the world, it comes to them all, just as, eventually, it will come to the very planet.
Yes, Death has known Harry Potter his entire life, and chose him for His Master long before Harry knew of the Hallows.
He chose him when those green eyes looked up at Him as Tom Riddle Jr turned his wand to the boy, as the tiny boy raised his chubby arms up and smiled at Death while Tom Riddle Jr spoke those two words that would take the life of a child, a child who accepted Death openly, willingly, with a smile and trust.
And Death sat with that child as He twisted a Mother's sacrifice into a protection that would last a childhood.
He flew alongside a motorbike as they journeyed across a rejoycing people, He cradled a sleeping boy while the moon was high and sang lullabies as he was left to cry alone in his cupboard.
He watched as childish drawings and writings filled the small space and made it their own, He petted sweaty hair as the child suffered his illnesses without care or kindness.
Death gathered him up after Tom Riddle Jr's host fell to the protection Death weaved of Lily Potter's sacrifice, held him and whispered stories that Harry was not ready to remember, one day, when Harry was old and grey, when time was no longer his friend, when the last of his loved ones were collected.
Only then would Death reveal Himself, only then would He finally be free to collect His heir.
Death comes to all, even Death Himself eventually.