Thank you so much for writing this. This is my OTP of the series, and I love how you stuck true to the two of them. Personally, they work for me because no one looks out for Harry the way that Severus does, no one is there when Harry needs them the way that Severus is, and no one reminds Harry that the world is not black and white as much as Severus Snape.
They work because they both know the darkness and horror of an abused childhood, war, and the need for someone to reach out and love them.