On the night of Lily Potter's death a charm activates in regards to Severus Snape. Its purpose is to return something very precious back to Severus, his memories, their memories. Severus Snape/Lily Potter

Snape doesn't hate Harry James Potter, not necessarily. If someone was to be accurate they would need to say that he hated what the boy represented. Harry was the embodiment of the greatest mistake of Severus Snape's life. Severus Snape/Lily Evans